Better recurrence instance handling and wallentry management.

[buddh4]

Recurring root event wallentry

In the latest version I've removed the wallentry of recurrent root events, since the wallentry of the recurrent root event is identical to the wallentry of the first instance. A recurring root event wallentry should rather contain information as:

xy created a recurring event starting at xy repeating every year

E.g. by using https://github.com/simshaun/recurr#rrule-to-text

Recurring instance creation

There currently is no well defined rule when recurring instances are initialized. Recurrent instances (and the related wallentries) are created when:

• Clicking on the event in the calendar
• Reminder process which searches for upcoming events and initializes recurrent instances

There should be a cronjob which automatically creates all upcoming recurrence instances of a month. Furthermore the
next recurrent instance should be created, once the last event has finished.