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.
Ah, so we're not actually proxying the image on the content server. We make a xhr call to the profile server (which can be direct for browsers that support it) and the profile server streams the image back in base64.