Remove the 'email' and 'mobile_phone' fields on Person and use identities to represent this data instead.

- Migrate data on existing Person objects to Identity objects
- Make the 'is verified' logic only accept certain ID types
- Sync the Person.is_verified and API 'verified' filter logic

- 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.