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.
If this is created in app-start, can we pass in config and ditch Session? - See https://github.com/mozilla/fxa-content-server/pull/1584/files#r17433326 for more context.