Serialization has been moved to serializer classes, sharing them when
sensible and Views have been converted to ViewSets to simplify code.

Adding tests for a serializer prompted moving of view tests to their own
subdirectory.

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

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

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