Watchlist: Separate non-form parts out of the <fieldset>

These include the information blurb at the top and the "Mark all pages
visited" button.

This also allows us the change the form-inside-fieldset nesting to
saner fieldset-inside-form. The 'mw-watchlist-form-namespaceselector'
id was removed; the primary form now has id of 'mw-watchlist-form'.

Bug: 48615
Change-Id: I9398922dc4cabc7b190f8d97c5d91a05710206bb
Merge "Watchlist: Separate non-form parts out of the <fieldset>"