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.
The config is actually mocked out. I set the URLs directly on `Session.config` and the clients pick them up, so, barring a regression, changing the URLs will Just Work. I should add a note however, because they are/look like our configured URLs.