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.
Ugh you're right– who uses ftp anymore? We can probably cut that. Valid domains may start with `-.` according to the  [reference](http://mathiasbynens.be/demo/url-regex).