User invite routes in admin module

When a new user is invited send them an email with a sign up link in.
Add registration screen for invited users

This allows users with an invite to sign up for popit and start editing
the instance they've been added to.
Fix registration for on legacy upgrade page

The registration app is only running under the hosting app, so we need
to explicitly add the hosting site's base_url in the form action.
Remove password confirmation from legacy upgrade

There isn't a password confirmation on the registration page, so this
makes it consistent.
Fix invalid markup on admin page

The remove person modal needs to be inside the li tag for the markup to
be valid.
Handle inviting people with existing accounts
Show invited users on the admin page

These need to be iterated separately to the permissions as they have
different properties. For now I've just repeated the email address twice
since we don't know the users name at this point, but the design breaks
if we don't include something in both tags.
Remove invite after user successfully registers

An invite code can only be used once, so we should remove it once we've
successfully signed the user up and given them permission to access the
instance.