Move WatchedItem logic to WatchedItemStore

Move WatchedItem logic to WatchedItemStore

This also removes assumptions that when a page
in one Namespace should be watched / removed
that the page in the talk / subject ns for the
page should have the same action applied

This should maintain all backward compatability
for the WatchedItem class

This also includes tests written by:
 - WMDE-leszek
 - Addshore

Bug: T127956
Change-Id: Iad9abafe4417bb479151a3bfbee6e1c78a3afe3c