Add an avatar builtin file generator

Summary: Ref T10319. This builds out a reasonably decent avatar generator. 256 colors x 74 images x 2 borders, 38k options. Not completely sure though how names disburse though, so likely half that number. I can add lowercase lettering to double the footprint if needed though.

Test Plan:
UIExamples. Color generator here: http://tools.medialab.sciences-po.fr/iwanthue/

{F3416622}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10319

Differential Revision: https://secure.phabricator.com/D17418