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.
This is specified [up above](https://github.com/mozilla/fxa-content-server/pull/1496/files#diff-33b4093ed4c55b70f6445f915776ac26R24) and probably isn't necessary here (or vice versa).