Implement recurring events

[Jelmer van der Linde]

Currently the iCal feed seems to work quite nicely. But to replace the Gcal calendar completely recurring events like the dombo and monthly borrel should be added.

To support these I propose we implement support for recurring events. To make things more easy we may use the conventions of webcal's RRULE attribute directly. One simple way to do this may be through this PHP library: https://github.com/tplaner/When/tree/develop

We could also do something simpler but we need to support:

• Monthly recurrences (e.g. first wednesday of the month)
• Weekly recurrences (e.g. every thursday at 4 o'clock)
• Yearly recurrences (e.g. every 20th of September)
• Exceptions (e.g. we may want to move or cancel a borrel once in a while)

[Jelmer van der Linde]

We now just copy the recurring events from Gcal and use their interface and logic. Done!