If the notification to be published is regarding a Role, Identity or
Consent object, we now add the 'data' field with a 'person_id' key where
the value is the id of the Person connected to the Role, Identity or
Consent.

Resolve: GREG-64

- 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.

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

Resolves: GREG-63

Enables rollback to previous version of a Consent Type in the history
part of the admin interface.

- Takes OrgReg url and api key as arguments
- Assumes that the OrgReg api has been registered behind a Gravitee
  instance.
- Introduces active and deleted fields to OrganizationalUnit for
  keeping track of status in OrgReg. Logic should be introduced in the
  future to ensure that Sponsors and Guests are not connected to
  deleted or inactive OUs.
- Add search and orgreg_id to Admin interface of OUs.

Useful for checking that there are consent objects for a certain
person for all the mandatory consent types.

The custom repr and str methods of various models now have tests.
Duplicate fixtures have been moved to conftest.py to reduce clutter.

All fixtures have been modified to create database objects, and then
fetching them back afterwards so that we work on the data saved in the
database, instead of the data as it is on input.

The health endpoint now has a test

Issue: GREG-26

Issue: GREG-26

Remove many-to-many relationships from the Person model,
keeping the 'through' models which continue to work as before.

Issue: GREG-26

Type checking is now also done for management commands and the usual
missing imports errors are ignored.

Issue: GREG-26