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.
Not for this PR, but I also think we should merge the ERROR_TO_CODE and CODE_TO_MESSAGES tables. The numeric IDs are duplicated and it's hard to just scan through the tables and find what you are looking for.