Start dates can now be blank, check for correct sponsor has been moved
to the more appropriate validate method and we now check that the role
exists before doing anything in the serializer

If the user is neither a guest or a sponsor, generic text explaining
this is shown. If they are a guest, they are shown the data related to
them. If they are a sponsor, they are redirected to the /sponsor/ route.

Resolves: GREG-97

To prevent hammering on the invite endpoint we throttle requests to the
endpoint to 100/day per ip.

Resolves: GREG-84

To prevent the invite id from showing up in various logs along the way,
we change to use an anchor in the invite url, and a post request to the
backend.

Invite links are now on the form /invitelink/#<uuid> and the frontend
redirects to /invite as usual after the session has been created.

Resolves: GREG-90

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

Code for guest routes moved to their own folder so that we don't need
multiple api calls.
Serializer class for roles has been simplified with methods for
validation of each field and a separate one used in invites without
requiring the person field.

Resolves: GREG-61

Allows search using e-mail and mobile phone, limiting results to 10
first matches

To make handling info about roles and persons easier we split them into
separate objects. The Profile page has now gotten its own api endpoint
where a sponsor has access to all guests instead of only those they are
sponsor for which is used on the sponsor frontpage. This is necessary
since a sponsor may want to give a role to someone they are not
currently sponsor for.