This subtask is for developing the underlying notification data and the methods for importing and refreshing that data. There will be nothing to QA against it until we build UI utilizing this data, so this task can move to PM signoff once development is complete.

The API call to fetch data is here: https://en.wikipedia.org/w/api.php?action=query&meta=notifications&notwikis=enwiki&notlimit=max&notprop=count|list&notformat=model

We must make the corresponding call (change subdomain and notwikis param) for each of the user's app languages, plus common others (wikidatawiki & commonswiki).

• Initial work - new database migration, some code cleanup, removing legacy polling, capturing additional notifications data into managed object, fetching first page.
• Instantiate fully realized cell view model from RemoteNotification managed object properties.
• Refactor fetching first page operation into one import operation, and another refresh operation. Importing will need to continue paging until all pages have been exhausted as indicated by continue in response. Refresh operation will need to continue paging until the oldest object returned already exists in the database. Use a background managed object context for saving these notifications. (Partly done in https://github.com/wikimedia/wikipedia-ios/pull/4047 and https://github.com/wikimedia/wikipedia-ios/pull/4048, needs NSNotification posting).
• Fetch additional unread notifications from non-app languages. First make fetch call with notcrosswikisummary param (see "API note" section below), then fetch unread notifications from each new project from the crosswikisummary object. (TBD) Note we might need to filter these by recognized language types, so we aren't importing notifications from the rarer notwikis projects. ) (Note: we do not need to fetch read history on these types of notifications. Otherwise that would complicate when exactly to trigger an import since any language with unread notifications could bubble up at any time.)
• If user changes app preferred languages and adds one, detect which ones are new and kick off import process on new languages.
• (optional) If user changes app preferred languages and deletes one, clean out data in database housekeeper.
• Prevent import and refresh operations from executing multiple times back to back. Only one import or refresh process should happen at a time. Also guard against the import processing occurring unnecessarily for a project we have already imported. Persist which projects have been imported, and only allow refreshing once they are imported.
• Allow import operation to continue importing where it left off if the app gets terminated mid-import. This might involve persisting continue identifiers.
• Bulk delete all notifications from Core Data upon logout, as well as any other persisted data we needed to keep track of like continue IDs and imported boolean flags.
• (optional) Move import trigger from notification center appearance to login in the app lifecycle.
• (optional) Add separate ability sync read and unread flags periodically. There's a chance someone could flip a read flag for a notification deep in history that we have already imported, and thus may not touch again in the refresh step. For this step, we could only fetch unread notifications from each project, be sure unread flag is updated in the database. Pull any local unread notification that did not return from this fetch and flip to read. This is a nice-to-have, we've confirmed that history doesn't need to be totally in sync at all times.

Other useful links:

Notifications API documentation: //www.mediawiki.org/wiki/Notifications/API
Original spike findings: https://phabricator.wikimedia.org/T284239

API note

NotCrossWikiSummary call to make to gather additional unread notifications of interest.
https://{primary language subdomain}.wikipedia.org/w/api.php?action=query&meta=notifications&notwikis={primary language wiki}&notlimit=1&notcrosswikisummary=1

The summary object has an ID of -1 and will return an object of foreign wikis that contain unread notifications.

Example summary object:

{
                    "wiki": "enwiki",
                    "id": -1,
                    "type": "foreign",
                    "category": "other",
                    "section": "all",
                    "timestamp": {
                        "utciso8601": "2021-07-21T17:17:38Z",
                        "utcunix": 1626887858,
                        "unix": "1626887858",
                        "utcmw": "20210721171738",
                        "mw": "20210721171738",
                        "date": "21 July"
                    },
                    "agent": {
                        "id": 35904678,
                        "name": "TSevener (WMF)"
                    },
                    "targetpages": [],
                    "*": {
                        "header": "More notifications from 2 other wikis",
                        "compactHeader": "More notifications from 2 other wikis",
                        "body": "MediaWiki and Test Wikipedia",
                        "icon": "global",
                        "links": {
                            "primary": [],
                            "secondary": []
                        },
                        "iconUrl": "/w/extensions/Echo/modules/icons/global-progressive.svg"
                    },
                    "count": 2,
                    "sources": {
                        "mediawikiwiki": {
                            "title": "MediaWiki",
                            "url": "https://www.mediawiki.org/w/api.php",
                            "base": "https://www.mediawiki.org/wiki/$1",
                            "ts": "2021-07-21T17:17:38Z"
                        },
                        "testwiki": {
                            "title": "Test Wikipedia",
                            "url": "https://test.wikipedia.org/w/api.php",
                            "base": "https://test.wikipedia.org/wiki/$1",
                            "ts": "2021-07-19T16:28:52Z"
                        }
                    }
                },