Parent ticket https://phabricator.wikimedia.org/T339334#9092727
In order to be able to store per-wiki statistics, introduce a `WikiSiteInfo` Eloquent model and add such info to the `Wiki` model, using a one to one relationship.
The fields of the `WikiStats` model should match the info returned from a `siteinfo` MediaWiki API call (dropping irrelevant items like number of jobs):
```
"pages": int,
"articles": int,
"edits": int,
"users": int,
"activeusers": int
```
Also. a boolean field `isFeatured` is introduced at `Wiki` level.
AC:
- [ ] The new `WikiSiteInfo` model is created and attached to the `Wiki` model
- [ ] The new `isFeatured` property is created on the `Wiki` model, defaulting to false
- [ ] Migrations are created
- [ ] Applying the migration created all-zero data for each wiki