We had a problem that became more evident to me in the newly-introduced
History tab in the Git panel where the avatars wouldn't show up for a
long time. Problem was that we were trying to render the avatar before
the GitHub user email came in, and that wouldn't work well because it
would 1) skip the fast CDN (which could get rate-limited fast), and 2)
cache the `None` result. So this improvement works by only attempting to
render the avatar when the email is available.

Release Notes:

- Git UI: Improve the display of user avatars in Git-related surfaces.