refactor: rework calendar

Up until now the calendar kept a single record of calendar events. This
record was modified whenever new or other events needed to be displayed.
Events were added/changed/removed imperatively (i.e. when
entering/leaving conflict mode), which goes against the principle of
unidirectional data flow embraced by Vue and other reactive frameworks.

The calendar is now split into separate components most importantly the
normal week view and a separate conflict view with separate state
management. The state management itself has been reworked and improved
so that it uses unidirectional data flow.

These changes massively improve performance, simplify state management,
and should make it easier to reason about the code (conflict mode was
hard!).

fixes #287