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'm not sure if we care, but there could be a subtle memory leak here with these handlers since the closure references the image, creating a circular reference. GCs are intelligent when dealing with circular references on normal JS objects, but there used to be problems with circular references when DOM elements are involved. Not sure if that is still the case.