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.
Can this sequence of creating an OAuth client, generating an assertion, and creating the profile client be abstracted into a function or moved somewhere else? The setup steps obscure what you are trying to do, which is request a remote image from the profile server.