Add integration with 'sharebetween' module

[dantefromhell]

Based on conversation in humhub/sharebetween#57

This is a feature request to implement an integration with the sharebetween module.

Current situation:
When a calendar event is shared from Space A to Space B, only the stream post is copied/ linked.
The event itself is not copied/ linked in the calendar of Space B, leading to issues like

• The event not showing in any calendar view in Space B
• The event not showing in the Upcoming events widget in Space B