BadFileLookup to replace wfIsBadImage

BadFileLookup to replace wfIsBadImage

I think this probably shouldn't be directly in the MediaWiki namespace,
but I don't know where is a better place to put it.

In order to avoid gratuitous use of TitleFormatter, I changed the cache
format -- the old implementation used getPrefixedDBkey() and I switched
to an ns/dbkey pair. I also changed the cache keys to use SHA1 instead
of MD5, by Daniel's request.

The previous implementation cached the parsed blacklist for one minute
without invalidation, so it could return slightly stale results, but it
didn't retrieve the bad image list message on a cache hit. The new
implementation unconditionally retrieves the bad image list message, but
uses a hash of it in the cache key and caches for one day. The new
behavior happens to be more cleanly implementable in a service.

Bug: T200882
Bug: T139216
Change-Id: I69fed1b1f3cfc1aa149e0739780e67f6de01609d