[WIP] Add wbgetmediainfo API module
This is basically a shortcut to wbgetentities that allows the
API user to get a MediaInfo entity without querying for the
MediaInfo ID (either by getting the page info and looking at
the page_id, or by querying the new page_props entry), and using
the file page title instead.
n.b. https://gerrit.wikimedia.org/r/382603 must be merged first.