This introduces an ApiWatchlistTrait that refactors out common code
across APIs that allow you to watch pages. Some methods have been
migrated from ApiBase and changed completely, but codesearch suggests
they aren't being used outside the API modules in this patch.
Bug: T248512
Bug: T248514
Change-Id: Ia18627b9824dca81f44f0571e8420d89b7626cf6
