Data: Add a way to create invites

Invites are the cool, new way to
ask someone to join a circle, so
let's make an endpoint for making
them.
Admin: Add button to create invite links

Create invites that allow for one person
to use them.

Also add some clarifying text around what
exactly the 'add member' section does.
Add invite page

Add a page to accept invitations. Also
implement about half of it. Specifically,
accept invitations by creating a new
account.
Refactor: Move signIn() into lib

Prepare to sign in from the invite page.
Refactor: Move goHome() into lib

Move the post-sign-in function that
goes to an account's default circle
into the lib service.
Invite: Sign in after creating an account

... and other loose ends around moving
the sign-in process into lib.
Invite: Accept with existing accounts
Data: Do not allow duplicate memberships
Invite: Set active circle to circle joined

... after signing in.
Refactor: Move goHome() back into SignInCtrl

It turns out the functionality in goHome()
is specific to the sign-in page, and we
don't want it in the invite page.