fix(avatars): add profile server client to proxy remote images

Fixes #1458.
Fixes #1447.
Fixes #1469. It fixes the client portion, at least. The server depends on https://github.com/mozilla/fxa-profile-server/issues/31.

Not sure if we want to merge until the server side is ready.
Any more of these weird RegExps and I'm going to think that we'll need a module we can start sharing between projects. Between this and the funky email validation code that we have sitting around...