Additional recurrence frequencies

Problem/Motivation

Default list of recurrences (day, week, month, year..) is limiting for some types of events.

Steps to reproduce

n/a

Proposed resolution

The default list should be extended to include Quarterly, Half yearly, biweekly, fortnightly etc.
OR
Allow user to define a custom frequency

Remaining tasks

n/a

User interface changes

The event creation form should provide more options

API changes

n/a

Data model changes

n/a