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 will need to be updated to pass an fxaClient into Assertion like here: https://github.com/mozilla/fxa-content-server/pull/1645/files#diff-813d67e41b2c362b78cd26f00841d96fR125. Depends on which merges first whether this PR will need to be updated.