Auto-generate profile images for sad psyducks

Summary: Fixes T10319. This looks for custom profile image, then falls back to a generated profile image.

Test Plan: Create a new user, log in, and see new profile image. Note this seems to break bin/lipsum generate user

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10319

Differential Revision: https://secure.phabricator.com/D17467