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.
But the good news is that this seems to support fancy unicode-esque URLs:
```js
var urlRegex = /(https?|ftp):\/\/(-\.)?([^\s\/?\.#-]+\.?)+(\/[^\s]*)?$/i;
urlRegex.test("https://persona.org/"); // true
urlRegex.test("gopher://persona.org/"); // false
urlRegex.test("http://mañana.com"); // true
```