fix(avatars): integrate the profile images backend

We shouldn't merge this before the dev backend is ready: https://github.com/dannycoates/fxa-dev/issues/24.

Fixes #1556.
This looks a lot like https://github.com/mozilla/fxa-content-server/pull/1584/files#diff-4170b02cafae3656dba3a0892df9e0dbR102

Should it be some helper-type function somewhere?