Message sent out should be an invite.ics not an appointment.ics

[DoubleMalt]

Rationale:

In many integrated email/calendar clients clients, an invite.ics will trigger a UI with a clear call to action to accept or reject the appointment and add it as potential appointment to the calendar while an appointment.ics only shows an easy to miss link to put it on the calendar that is easily missed.

Tested with Gmail and ProtonMail.

At least an option to send an invite.ics instead of an appointment.ics would be great. Nextcloud calendar also sends out invite.ics files that show up as expected.

[DoubleMalt]

I'm happy to look into it an submit a pull request if it has the chance of being accepted.