Add a global-wiki 'mark all as read' to the Special:Notifications page.
The 'mark all as read' will makr all notifications in the given
wiki. The context of the wiki changes when filters are chosen,
and so the message of the button changes as well.
Bug: T115528
Change-Id: Ibd9dcdf7072d6cbc1a268c18e558e6d0df28f929