Feature/invites

Here's my first take on the invitation thingy. Invitations section is rendered at the bottom of the account editing page (/account). Invitation currently works for people already in the system. Handling external people is still a TODO. Also, I kept the original inviteUser method so Admin invitation link does not break.

I'd love to hear your ideas on how to handle external people: send email/ask people to email an invitation link etc.