Resolves: GREG-173

- Combine the /invite and /invitelink endpoints into one
- Return error messages from the backend
- Handle errors in the frontend
- Lower number of queries ran when calling the userinfo endpoint

Does the same as before with less and more readable code.

Any user that does not have a GregUserProfile must be a superuser so
this is not a problem per se, but it should be handled. For now we
simply treat them as an anonymous user and deny access.

Resolves: GREG-116

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

- Expanded output from guest info api endpoint
- Share guest info between both routes under /sponsor of the frontend.
- More localised text options added for use on the sponsor pages
- Details button on the sponsor overview page now points to the guest
  info page for each person.

Resolves: GREG-74

Before this commit, the userinfo endpoint would fail if the user
did not have a person object connected to their GregUserProfile. We now
check that this is the case before checking the person objects roles.

- 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.

Issue: GREG-36

Issue: GREG-36

Issue: GREG-36