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.
Aye, one of the original tasks for #1327 (and in Ryan's design) was a full page loading screen, but since we didn't have a backend yet it wasn't a priority. We're missing an issue though– I'll file one.