refs #128

This allows us to visualize a continuous program in the week view
without the need to manually generate slots to fill empty spots in the
program.

refs #128

This highlights events with negative durations. There might be other
invalid event states that we could/should visualize so that they are
adressed.

These event states shoud probably be fixed in steering, but it might be
good to spot them if they pop up.

Timeslots might be cut as a result of conflict solutions. The start and
end time should therefore be part of the cache key.

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

Since we now use a setup component for the calendar the old comments are
no longer relevant and we can use a regular slot.

Now that bootstrap-vue is no longer used we can finally deactivate the
Vue 2 compatibility mode.

This is a big one because it’s hard to make these kind of changes
atomic. This changeset reworks the calendar component which is no longer
dependent on bootstrap and vuex.

As a consequence we can remove axios, bootstrap, and vuex because the
calendar was the last remaining part of the code that was still using
them.

refs #127 #140 #261 #279