Introduce image size bucketing

See the conversation on mediawiki.org:
http://ur1.ca/g14jv

We're trying this out as one option in combatting slow thumbnail load
times. It may or may not work very well.

Bug: 56695
Change-Id: If1211fdff87c0782c7355d654415bfd29d63d84a
Merge "Introduce image size bucketing"