Allow filtering by username on Special:NewFiles

Allow filtering by username on Special:NewFiles

Meanwhile, also change ApiQueryAllImages to search by
user ID instead of username. This is also how ContribsPages,
LogPager, ... work already.

This will use the new index on img_user. The old index is still
around, but is likely not going to be used in practice (users
have to logged in to upload)

Do not merge until I4db7bd0f917a428f40eb315246f89d49c7ae5b6a has
been deployed.

Bug: T125071
Bug: T121865
Change-Id: Ida22555f00530f393e739366fa9e79ecabf3c0eb