Make Special:MIMESearch a non-expensive special page.

This tries to use the index on (img_media_type, img_major_mime, img_minor_mime).
This is not the ideal index for this purpose (due to starting with
img_media_type, and not having an img_name at the end). However it
seems usable due to the low number of values img_media_type can have.
Thus we can just specify all values, and the index will get used
as a range index.

The only potentially problematic thing I see is that it uses
LIMIT X,Y for the offsets (due to being a query page) which can
be expensive on big offsets. If that's an issue, we could limit
the number of results to be at most $wgQueryCacheLimit or something.

Note: There's some discussion on the bug rather negative to this
special page in general. While I agree with much of the criticism,
I still think this special page can be useful, and if it can be
made efficient without needing to modify the indicies, we might
as well.

Bug: 13438
Change-Id: I88826ae7bff0812374157e596f86a19b248220e1