Currently, Wikibase accepts only one way for sitelinks. their canonical database name (their id in sites table).
It is not uncommon for WMF wikis to have their site ID changed
**Example: **"Belarussian Taraškievica" Wikipedia use to have a site ID `be_x_oldwiki`, which is also wiki's database canonical name. At some point the wiki has been renamed to `be-taraskwiki`. The `be_x_oldwiki` remained to be stored in the database as a "canonical site ID", but it is `be-taraskwiki` which is primarily used in the UI and expected in the user input (UI and API)
**Change requested: **There will be a configurable site ID aliases to be used in favour of "canonical" site IDs**
**Acceptance criteria:**
[] The canonical site ids are still used for storage in JSON and other indexes.
[] alias site IDs are the IDs presented to the user in the sitelink editing UI
[] One of the aliases is configured as the "label" to be shown in the UI - especially for cases when there are multiple aliases per "canonical site ID"?
[] alias site IDs are presented in the JSON output provided by Wikibase APIs (including Special:EntityData)
[] One of the aliases is configured as the "label" to be shown in the API output - especially for cases when there are multiple aliases per "canonical site ID"?
[] Configuration is open for more aliases to be added for other siteids
[] The canonical site ids are still used for storage in JSON and other indexes.
[] Configuration is documented in options.md
[] Default Wikibase configuration does not have any aliases
[] WMF production config is adjusted so that
[] `be-taraskwiki` is accepted as an site identifier when adding a sitelink to "Belarussian Taraškievica" Wikipedia (canonical site ID `be_x_oldwiki`)
[] identifiers containing underscores, as well as `be_x_oldwiki as still accepted as site identifiers when adding a sitelink to a respective Wiki
[] `be-taraskwiki` is displayed in the sitelink editing UI when adding a sitelink to "Belarussian Taraškievica" Wikipedia
Originally part of T114772