Greg 90 safer invites

See merge request !133

Introduce OuIdentifiers

See merge request !128

Greg 73 mail templates

See merge request !132

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

Even though start_date can be optional in the payload, it will always be
present in the form.

Introduced to make editing of templates possible using the admin
interface. There are currently two types of templates, and there can
only be one instance of each type so that the system knows which
template to use in which situation.

Resolves: GREG-73

Fixing bug where details button was not shown

See merge request !131

GREG-100: Cancel invitation

See merge request !129

GREG-91: Expand sponsor api

See merge request !126

Greg 61 role button

See merge request !124

Fix script for populating db with test data

See merge request !127