To facilitate multiple identifiers of OUs we introduce the OuIdentifier
model and removed the orgreg_id field from OrganizationalUnit.

All current orgreg_ids are recreated as OuIdentifiers with name
'orgreg_id' and source 'orgreg'. Both values are configurable in the
base settings file in the gregsite app. Note that changing this value
after migration will require manual work changing all OuIdentifiers
already created before the next run of the orgreg_import management
command since that assumes all current orgreg identifiers uses these
values.

All endpoints that previously exposed the orgreg_id field now exposes an
identifiers field with a list of OuIdentifiers connected to that model
with fields id, name, source, and value.

Resolves: GREG-88

- 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.

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.

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

Issue: GREG-26

Fixes: GREG-13