Calendar functionality

[Kristina Hoeppner]

from https://mahara.org/interaction/forum/topic.php?id=7945&post=31930

Add a calendar feature in groups. Adding external calendars like Google Cal is not an option.

Here is a draft list of functionalities and features:

* Activate Calendar as function for a group
* Define who can add calendar entries: group admins only | each member of a group
* Add calendar entry: one time | repeating entry (daily, weekly, monthly, annually)
* Title, description, start date, end date, location
* Status: planned, confirmed, done
* Connect with member of the group: this date is booked for...
* Confirmation by added members
* Edit/delete entry
* Show entries in monthly, weekly views
* ical/ics export
* Import entries from excel: format dd/mm/yy and hh:mm