Remove usages and hard deprecate the following User methods:
- addWatch(): use WatchlistManager::addWatch() instead
- removeWatch(): use WatchlistManager::removeWatch() instead
- isWatched(): use WatchlistManager::isWatched() instead
Usages in core have already been removed except for in User tests.
**Wikimedia production extensions/skins:**
[] Echo
[x] LiquidThreads
[x] FlaggedRevs
[] VisualEditor
[] Wikibase
[x] WikibaseMediaInfo
[x] MinervaNeue (skin)
[x] Nostalgia (skin)
**Other extensions/skins:**
[x] BlueSpiceSocialWatch
[] CreateAPage
[] MediaWikiAuth (master compatibility policy)
[] PageForms (master compatibility policy)
[x] PushToWatch
[] TranslateSvg
[] Video
[] WhoIsWatching
[x] Apex (skin)
[] Metrolook (skin)
[] Nimbus (skin)
[] Strapping (skin)
[] WebPlatform (skin)
[] edwardspec / mediawiki-moderation (github: https://github.com/edwardspec/mediawiki-moderation/pull/63)
[] ~~OAuthAuthentication (incompatible with MW 1.35+) ~~
**Core:**
[] Hide deprecations in User tests
[] Hard deprecate User::addWatch() and User::removeWatch()