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.
Yep, good idea. We'll might need a profile server wrapper like we have for the fxa-js-client once we support more calls. But I can move it to a function for now.