Resolve single activitymoment outside of recurrence issue

[DutcherNL]

The new activitymoment committee page exposed an issue in approach around recurrences. A shift needs to be made from Activity as root of recurrences to Activity as umbrella that can have occurences, but also non-recurrent instances.

• Remove recurrence_id from non-recurrent activitymoment instances in calendar feed
• Fix get_activity_between for activitymoments outside of recurrence rules

If done well also should resolve #107