Story 1262: Adding user to last modified

A second pass at Kaldari's 1st attempt on this tricky piece
of coding. This one only adds the user profile with JavaScript which
simplifies the i18n messages somewhat. Features the most creative use
of PLURAL evah.

Also breaks out common functionality between lastModified in beta
and stable and ensures they do not run at the same time.

Minimal changes to the skin itself.

Also rename sibling-link class since it no longer always refers to
a link

Change-Id: I0874956d593c02bea38d45eafebbba781dc5ab9a