Support redirects in JavaScriptContent

Support redirects in JavaScriptContent

When a JavaScript page is moved, a "redirect" in the form of
mw.loader.load(...) will be left behind, so any other
JavaScript loading the page that way will still work, albeit
with an extra HTTP request.

This also implements Content::getRedirectTarget(), so redirects
are marked properly in the database, and users viewing them
are redirected properly. A magic "/* #REDIRECT */" comment
must be in front of the mw.loader.load call. This is done so
that pages which currently are just one mw.loader.load call
aren't turned into redirects.

Bug: 71200
Bug: 33973
Change-Id: I10fdff087a901da56fad64531f0e382f90ebcf37