Added the Magic Wormhole UI to the webapp for pairing Tor remotes.
This replaces the XMPP pairing UI when using "Share with a friend" 
and "Share with your other devices" in the webapp.

[[!img assistant/share_with_a_friend_walkthrough/enabletor.png]]

[[!img assistant/share_with_a_friend_walkthrough/wormholepairing.png]]

I have not been able to fully test it yet, and it's part of the `no-xmpp`
branch until I can.

It's been a while since I worked on the webapp. It was not as hard as I
remembered to deal with Yesod. The inversion of control involved in coding
for the web is as annoying as I remembered.

----

Today's work was sponsored by Riku Voipio.
