Use actual github avatar for user

Github user info contains an avatar_url which provides the avatar for
the user regardless of whether the avatar was uploaded directly to
github, comes from gravatar, or is a default provided by github.  This
uses that, so avatars uploaded to github are supported.

Currently only the logged in user; similar still needs to be done for
other places where github avatars show up
Fix bug in size selection of org avatars

We want ?v=2&s=25 rather than ?v=225
Use github avatars for build invites
Github avatars for followers in org settings
Non-hackish function for sizing github avatars

This decreases the changes that anything will break if github changes
the form of the avatar urls they provide.

cemerick.url updated to lastest version, for the cljs support

Deleted newly-unused gravatar-url function and previously unused
email->gravatar-url function.
Fall back to gravatar, and better changelog message
Merge pull request #3276 from circleci/github-avatars

Use actual github avatars
Merge commit '1e127e2ac73b70c460f2475c3ef2ea63df2f96fa'