Fallback further than gravitar when default

I generally dislike when users have no profile image. Because of that I would like to see if we could fall back further than just using gravitar with the csh email. I would like to see it fall back to use:
- Google account with gravitar if the user has added their Google account to profiles
- GitHub username profile image?
- See if we can grab RIT id photos

Start by reviewing the existing profile-image fallback path and how Google and GitHub accounts are represented; the issue names no files or tests. Define which sources are supported, how unavailable or private images are handled, and tests that confirm the selected fallback order.

Written by the indexing model from the issue text.