Do not allow sorting Special:Listfiles by img_name when filtering by username

We don't allow sorting by img_size due to lack of index. For consistency
we shouldn't allow by img_name either, since there is similarly
no index on (img_user_text, img_name) either

Also, let filtering by img_size when not in miser mode.

Change-Id: I5aaf1a3f39ddc23d5d009ada199b63a0e3133ef3