Add file usage data to MMV metadata panel

Interface code is its own class which does not depend on the main
interface class it can be unit tested (and eventually moved into its
own file to make browsing the code easier). IMO we should aim to
eventually break up the interface into similar classes (with a simple
init/empty/set interface + custom events where it makes sense).

Also, sneak-introducing LESS!

API usage could be more effective (globalusage is a separate API call;
it needn't be), but we will have to rewrite that part soon anyway, so
it should pass for now.

Bug: 60087
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/44
Change-Id: Ibe5c323cdeab4a378316925f0c3efb3dc7ef5997