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.
Can this be made in app-start and passed into the views instead? It'll make it easier to untangle and merge cleanly with https://github.com/mozilla/fxa-content-server/pull/1645, esp in regards to the fxaClient that will need to be passed into it.