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.
I realized last week that the overlap in errnos means @kparlante can't easily figure out which error was thrown without looking at surrounding context. We might have to either come up with unique identifiers or namespace errors when sending them to the metrics collector.