fix(avatars): add profile server client to proxy remote images

Fixes #1458.
Fixes #1447.
Fixes #1469. It fixes the client portion, at least. The server depends on https://github.com/mozilla/fxa-profile-server/issues/31.

Not sure if we want to merge until the server side is ready.
﻿These values are hard coded in several places in the tests but can vary depending on configuration. This makes me uncomfortable because the tests will only pass as long as the dev config does not change. Not for this PR, but something to think about.