Use the default gravatar image when an e-mail address is missing.
Review Request #9563 — Created Feb. 1, 2018 and submitted
When a user doesn't have an email address, gravatars won't work. A
previous change of mine changed it to not crash, but this still isn't
the most ideal. This change makes it so we return references to
gravatar's default image, rather than just empty avatars.
Ran unit tests.