Make Attendee tab read-only in Shared calendar

[georgehrke]

Invitations can only be send for your own calendars (or calendars that you are a delegate for).

Hence, the attendee tab of events in a shared calendar should be read only.
When creating an event and adding a attendees, it should not be possible to move it to a shared calendar anymore.

[miaulalala]

Related: #947

[pierreozoux]

I will change the categorization as a bug, and here is my rational:

User Alice shares calendar "Association" with write permission to user Bob.

Bob can now creates event. And when Bob creates events, he wants to invite Claire to his event.
So he goes to the "Attendees" tab, add Claire, and there are no errors.
So Bob expects that Claire received an email or a push (depending on her preference) and that the event is now in Claire agenda.

It is not, Claire doesn't have the event in her calendar, and she didn't receive nor a push nor a mail, and there was no error showed to Bob. This is unexpected, hence it is a bug!

[pierreozoux]

It is true that the documentation is quite clear about this bug, but still, this is not a reason.

[Blackclaws]

Its even weirder that an event cancellation is indeed sent to all invited participants even though they never received the original invitation. Also there is a good reason to have a shared calendar that is write for others for shared appointments. The solution should instead be that invitations are sent out for shared calendars.