Implementiing Rrule package (cherrypick test)

[thomassth]

Mostly #403 but I cherrypicked and cleaned up some parts. Reduced lines changed (mostly linting) to make things clearer.

Currently working:

• [x] can build daily or weekly recurring events.
• [ ] can build monthly events.
• [ ] can build complex (eg. multi days a week) events.
• [x] can delete all events in the chain.
• [ ] can delete single events.
• [ ] can delete this and

Don't merge before rrule changes are published, and most features are working / non-working features hidden and labeled.

[thomassth]

Cherrypick does not improve visibility as well as I hoped, and does not respect authorship. Closed until needed.