Issue: GREG-66

Issue: GREG-66

- Introduces the Invitation and InvitationLink models for keeping track
  of guest invitations and the links connected to them.
- Adds a post endpoint accessible by sponsors to create invitations for
  guests.
- Adds a get endpoint for the guests to review their invitation
  information.
- Adds a post endpoint for the guests to confirm their invitation with
  updated mobile_phone number if they want.
- Removes token field from Person since it is not used.

Just print mails to console instead so no one sends real emails on accident

Issue: GREG-42

Issue: GREG-42

Register page now fetches OUs the sponsor has access to.
Sponsor page now shows the sponsor's active and waiting guests.

Resolves: GREG-63

Issue: GREG-36

Issue: GREG-36

Issue: GREG-36

Issue: GREG-36