Figure out if we can remove github_avatar_url

[humphd]

In #3703 we're adding support for showing a user's GitHub avatar in the timeline. Previously, we added a field to the db to store this, but our current implementation is doing it dynamically (i.e, we only need the GitHub Username).

I wonder if we can remove this field, and all users of it. We need to confirm and understand the implications before we do this.