Enable us to render a spinner while the hook loads

Relabel the invitation resend button to make its purpose more clear.

Removed and introduced some borders. Added some text. Increased the size
of icons.

Resolves: GREG-142

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

The profile page of a guest now shows a verification button if the guest
has a passport or national identificaiton number that has not been
verified. Clicking the button shows a dialog, with a confirmation button
which triggers a PATCH request to the backend and reloads the page when
it returns.

Resolves: GREG-101