Replaces watchlist notification methods in Title and User classes:
* Title::getNotificationTimestamp -> ::getTitleNotificationTimestamp
* User::clearNotification -> ::clearTitleUserNotifications
* User::clearAllNotifications -> ::clearAllUserNotifications
New service has 67.90% code coverage with pure Unit tests; as well
as integration tests for the DeferredUpdates part
A follow-up patch will deprecate the replaced methods, as well
as document that the `UserClearNewTalkNotification` hook now only
provides a UserIdentity (typehint added in T253435 but until now
a full User was still provided).
Bug: T208777
Change-Id: I6f388c04cb9dc65b20ff028ece607c3dc131dfc5
