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.
I kind of want to refactor things and put all the errors into an `error.js`. The variety of error modules is starting to feel cluttered and potentially confusing. Follow up PR?