Handle IMIP party crashers / invitation forwarding

Use case 1 – forwarding allowed

As a Nextcloud user, I want attendees to be able to forward their invite to others, and these other people to become attendees.
When creating an invite I want to select that forwards are allowed. As soon as someone's email client sends my server a REPLY, that person's email will be added as accepted attendee, and a REQUEST is sent out. Other attendees are also informed about the new attendee.

Use case 2 – forwarding prevented

As a Nextcloud user, I don't want attendees to forward their invite to others, but they might still do it.
When creating an invite I want to select that forwards are prevented. When someone's email client sends my server a REPLY nevertheless, I will be notified and asked for approval. Only if I approve that person's email will be added as accepted attendee, and a REQUEST is sent out. Other attendees are also informed about the new attendee.
In case I decline the person will receive a cancellation.

Original report

Currently, the iMIP endpoint completely ignores party crashers (ATTENDEEs that aren't in the original list).

There is a provision for party crashers in the iTIP RFC. https://www.hjp.at/(en)/doc/rfc/rfc5546.html#sec_3.2.3

Either accept them (as sabre does) or send a CANCEL.

We should send a CANCEL to the party crashers instead of completely irgnoring them.