(bug 39297) Show a warning if thumbnails won't be animated

(bug 39297) Show a warning if thumbnails won't be animated

This shows a warning on the image description page if the image
is animated, but thumbnails won't be. This includes
gif images that are too big, but also svg images that are animated,
and APNG files.

The message used is file-no-thumb-animation, but will also
check for file-no-thumb-animation-<image extension> so that
admins can do per image type explanations. Gif files have a built-in
explanation that is slightly different (Since its do to resolution).
Ideally one would pass the resolution limit to the gif message,
but I couldn't think of a clean way of doing that. (Also might be
complex to explain to user. They aren't used to resolution as a single
number but as a width x height type thing).

Moves isAnimatedImage from ImageHandler to MediaHandler, so I could
safely use it from any handler class.

Change-Id: I42ee11d889e0c41de53d0951f55a4338ca55311d