refs #129

API object attribute names are now:

1. camelCased (#164)
2. use Id / Ids suffix for object references (#172)

vue-full-calendar is not the official Vue package of fullcalendar, is
only compatible with Vue 2.x, uses the outdated fullcalendar v3 library
and contains a bunch of huge dependencies like jQuery and moment.

The changes in this commit migrate the dashboard to fullcalendar v5 and
the official vue component. Apart from that there have been a few
refactors and fixes. These are:

* remove irrelevant stub event callbacks
* make view updates reactive instead of imperative
  (see removal of renderView method and introduction of renderWatcher)
* rename methods to better reflect their purpose
    resolve → enterConflictMode
    resolveEvent → resolveConflict
* emit events in modals instead of call to methods on this.$parent
* let → const where applicable to prevent accidental reassignments
* switch to for-of loops (from for-in) for better readability
* fix some typos

One other major change is that timeslot and collision data is no longer
mixed with event data for fullcalendar and is assigned to the
extendedProps property instead. As mentioned in fullcalendars
documentation on event parsing [1], any data that fullcalendar does not
handle on its own is moved into the extendedProps object with the
explicit recommendation to put them there right from the start. We
follow that recommendation.

closes #94

[1] https://fullcalendar.io/docs/event-parsing

The steering API has some deprecated attribute names that we should no
longer use. These are:

  dstart → first_date
  until → last_date
  tstart → start_time
  tend → end_time
  byweekday → by_weekday

These changes in naming have also been applied to variable names,
attribute names and translatations in the dashboard code in order to
avoid confusion.