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.
[here](https://github.com/mozilla/fxa-content-server/pull/1584/files#diff-e8bcfe428107ce33dbc222423506acd2R24) you use a quality factor of 0.9, is the difference intentional?