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.
Looking to the future, I'm hoping we can have something called `fxapi` which encapsulates auth and all the other apis we connect to. It's something we'd likely expose to reliers, but it's also something we can dogfood here.