mw.ViewPageTarget.init: Setup "source" links even if VE is not available

Follows-up ced2a8a which moved the tab layout to the server-side
and changed it to set up "source" tab and section links always
everywhere (even if VE would not be availabe in the namespace
or browser).

The JS logic (which continues to exist to take care of pages
cached before we moved it to PHP and/or to fix up pages cached
with a different configuration in the future) didn't do this yet
causing the "Edit source" tab to inconsistently appear on pages
for anonymous users viewing pages where VE is not available.

Change-Id: Ic575b3fcef17e636adaa338abc7748a4388ed9a9
Merge "mw.ViewPageTarget.init: Setup "source" links even if VE is not available"