[Feature Request] Calendar (Integration)

# Feature Request: Calendar (Integration)

## Is your feature request related to a problem? Please describe.
OwnCloud Infinite Scale has no calendar feature.

## Describe the solution you'd like
A calendar per user, group, and space (when enabled) that can integrate with other calendar applications (using CalDAV).

### File Behavior
I want the ability to drag a file into a space on the calendar. This action can prompt a calendar entry form:
- Title: enter title...
- Description: File URL...
- etc.

When the user submits the calendar entry, it is entered into the calendar.
- User: Personal calendar submission only affects the user's calendar.
- Group: Group calendar submission is placed on the Group calendar and propagated to the group's users' calendars. The user has the option to disable the display of Group calendar entries on their respective User calendar.
- Space: Space calendar submission is placed on the Space calendar and propagated to the group calendars. The group and the user can disable the display of Space calendar entries on their respective calendar.

The OCIS UI detects calendar entries with an OCIS Document URL in the description (or another metadata field) and displays a file preview in the calendar (rather than an entry with a URL description). However, this feature will only appear on OCIS UI's because other calendar applications will not have the context that the calendar entries referencing OCIS documents should be customized. So those applications will display a boring entry with a title, description (url), etc.

## Describe alternatives you've considered
- No Calendar.
- Calendar as an integration rather than in the basic OCIS installation.

## Additional context
I am willing to develop a calendar integration for sponsorship or salary now that https://github.com/owncloud/ocis/issues/6503 is completed.