Special:WatchlistLabels: Set page subtitle navigation

Update the page navigation for legacy skins to include 'Manage
labels'.

This moves the SpecialEditWatchlist::buildTools() method into
the WatchlistSpecialPage trait, so it can be used in all three
places.

Bug: T411842
Change-Id: I96734eb4744291c382b7b3ad6fab998e79ef5b60