Add Special:MediaStatistics page for file type stats

Add Special:MediaStatistics page for file type stats

There used to be a bot on commons that did this, but it broke, and
this seems like the sort of thing that should be built in to
MediaWiki anyhow.

In order to do this, it abuses the querycache table somewhat
(Storing non-titles things in the title column).

Also changes the link on Special:Statistics for number of
uploads to link to new page. It seemed appropriate to link
to more detailed statistics instead of to Special:ListFiles.

Change-Id: I9ab768584b02a32b450d5f3981ff775ee07fecfa