Implement import profile picture from Gravatar

Summary: Add a field where you can put the gravatar email address to pull an image for the profile picture from

Test Plan: Tried uploading a file, replacing with default, and various combinations and they all still work.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2105

Differential Revision: https://secure.phabricator.com/D4809