Invite friends

This adds the ability to invite friends via email. We've decided it really only makes sense in the context of a party, so now when clicking the "invite users" button in your party you'll get a modal that looks like:

![img](http://i.gyazo.com/12a058704aff1506ac26126013e4a9e9.png)

* If new user: they'll click the link, which sets the `partyInvite` session object for later consumption after they've joined the site. Upon registering, that session object will be consumed and they'll be invited automatically to the inviter's party.
* If existing user, the same setup happens - but upon logging in (or simply visiting, if they're already logged in).
* When (if) invitee joins the party, inviter is rewarded 1 `Basi-List` quest scroll.