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.
Okay cool. I figured it was something like that. My opinion is that it encourages inconsistent use without a lot of actual convenience, but that's just my opinion :smile:.