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.
To be clear, the Assertion module aliases `Assertion()` to `Assertion.generate()` and the line above is resetting `Assertion.generate` after it was overwritten in `beforeEach`.