Add option to preload flows to FlowRegistry

This makes it easier to reproduce production-like
behaviour around memoization in tests.
Add failing integration test for storing calendar dates in memory
Add the CalendarState class

This will evaluate the calendar and store the dates
for the current request.
Move state out of the Calendar class
Invoke calendar methods for state and rendering separately in the presenter
Move the to_ics method into CalendarState

This allows us to make the code cleaner in the
presenter and controller.
Merge remote-tracking branch 'origin/fix-calendar-date-leaks'