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.
Not sure if 0.9 counts as a magic number. I vaguely recall seeing an 0.8 somewhere too.
https://github.com/mozilla/fxa-content-server/pull/1584/files#diff-a08f19adbb26f61c68aeb3013de3c5a6R80

Same may apply to "image/jpeg", which appears 9 times in this PR. Not sure if that should be variable-ized or extracted in the off chance you want to make the default a PNG or GIF in the future.