Make thumb.php give http redirects if given a file redirect

Make thumb.php give http redirects if given a file redirect

See bug 22390 for some of the background. Basically there's two
reasons why we want this:
* Hotlinkers to commons. We don't want to break their links when
we move a file (Wikimedia blog is a prominent hotlinker)
* Cached entires. On wikis using commons files, a file move doesn't
trigger HTMLCacheUpdate jobs for the downstream wiki (it only
triggers it on commons). This means that these pages will still
use the old version of the html (with img tags that have the old
url) until the next edit or purge action. But these urls won't
work as soon as the file is moved.

I'm pretty confident that redirects will work at this point
in the thumb stack, as the redirects for long file names work
fine (provided no thumb exists in swift with the wrong name).

Change-Id: I40350121cf902f66fcbd3cf788335a988fa7ee33