Allow custom image generation when choosing a profile image

Summary: Ref T10319. This swaps the default in the Picture Chooser to allow picking of the custom unique avatar. We're currently going with 100k unique possibilities. The logic roughly hashes a user name and picks an image pack, color, and border. Based on that, we select the first character of their username, or fall back to Psyduck if not [a-z][0-9].

Test Plan:
Set the following usernames from ProfilePicture as a test: chad, epriestley, sally, 007, _cat_, -doggie-.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10319

Differential Revision: https://secure.phabricator.com/D17430