We disabled preflight earlier so we can safely enable the base layer
again.

We often find ourselves adding flex styles to buttons. Instead, this
should be the default.

Although the docs [1] specify a class on Popover components, this
doesn’t seem to be passed down. We help ourselves with a selector for
the data-headlessui-state attribute .

[1] https://headlessui.com/vue/popover

see: https://tailwindcss.com/docs/preflight#disabling-preflight

refs #156

see 3c5042e5

We currently don’t include the tailwind base module because it would
interfere with the bootstrap styles.

Unfortunately the tailwind base module also contains a lot of CSS
properties that are used in various utility classes to allow
fine-grained manipulation like `tw-translate-x-2` which would only set
the `tw-translate-x` CSS property and all the real work happening in the
`tw-transform` class which applies all these properties in a single
`transform` rule.

As we can’t include the tailwind base module because of said conflict,
we should at least define some of the missing CSS properties.

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only compatible with Vue 2.x, uses the outdated fullcalendar v3 library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

Making imperative DOM API calls to apply CSS classes is
counter-productive when using a reactive frontend framework like Vue.
DOM attributes should instead be assigned based on reactive data.

fixes #117 and #118.

Notable changes:
 * Vue 2.6 to Vue 3 + compat migration.
 * Build system migration from vue-cli/webpack to Vite.
 * Application dependencies like bootstrap-vue, vue-router, vuex,
   or vue-toast-nofitication have been updated, if doing so was
   necessary or advisable for the Vue 3 migration.
 * Static file includes for the oidc-client in oidc_callback.html
   and oidc_callback_silentRenew.html have been removed in favor
   of automatic processing in the build system so that all assets
   are versioned and invalid cache hits are prevented.

Fixes #53 and #110.