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.
Is there a limit on how large this can be? Since this ends up [stored in the `Session`](https://github.com/mozilla/fxa-content-server/pull/1496/files#diff-b17c8962986cd808cdca186d692bcf73R102), is there a chance we could consume all the local/session storage with just the image?