consent & proxying end-to-end working via polymer

Consent and Proxying finally "works" through the new Polymer UI.

- This particular version is still a bit wonky and unstable - there are some conditions where get/give toggle and some data bindings don't always sync up. Usually re-opening the popup restores everything, but there is a bit more work to do to make the entire workflow actually robust.
- Scrolling through multiple instances work a little better in the UI. Also some style tweaks.

(Also, login/logout still only works with an older version of `freedom-social-xmpp`, so we should bump that version back up once fixed)

Tested: Jasmine & manual end-to-end.
Is there be an equivalent for moz / firefox?