Per-user settings for shared events

I would like to open a ticket concerning the functionality mentioned in #679. @georgehrke quoted the CalDAV sharing standard like this:

In section 5.5.4 it says:

Servers MUST support "per-user" calendar data in calendar object resources stored in shared calendars. This allows each sharee and the sharer to store their own alarms and free busy transparency status without "interfering" with other users who also have access to the same calendar object resources.

However, in the related PR #6903 only the minimal solution of silencing shared read-only calendars was implemented if I understand correctly. I would like to come back to this "full feature set" because of the following use case - which I suspect is very common:

• I am subscribed to many calendars (read-only) with seminars, lectures, meetings
• Some I have to attend or find interesting, so I would like to
  • set a reminder for them
  • set my own status to "busy" during these meetings

Right now I basically have to create a duplicate event in my own (writable) calendar to be able to do this. The cherry on the cake would be if this would also work for WebCal subscriptions.

Start by reading the requirements in #679, the implementation context from #6903, and the referenced CalDAV sharing standard. Done should support per-user alarms and free/busy status for events in shared read-only calendars, with WebCal subscriptions identified as a possible extension.