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

* deactivate buttons and show progress indicator while saving
* place primary button first
* add spacing between buttons
* add support for custom labels for cancel and save

The FormGroup’s save-indicator would jump around when used in
combination with the FormTable. We now have a more tight coupling for
these components so that the FormTable can influence the FormGroup’s
rendering.

This refactors the existing state helpers (useCopy and
useUpdatableState) and unifies them to useUpdateBehaviour, useCopy,
useAPIObjectFieldCopy, useRelation and useRelationList. This eliminates
a lot of custom code in the show settings page and unifies the behaviour
of all input fields and save operations making the experience more
consistent and understandable.

It also enables us to show the save-in-progress state and errors that
occurred during updates for all fields along with customizable debounce
and proper v-model support.

We also got rid of the remaining uses of useSelectedShow. The show
settings page still made use of it despite using a route that
provides the relevant show that should be edited.

This refactor includes a bulk of the changes discussed in #181. It
removes the existing navbar and replaces it with a sidebar instead. This
leaves us more room for context-aware navigation options and other
information that is relevant to users.

Additionally, the show management page is now split up into two, the
show episodes management, listing the coming shows/timeslots and the
show’s schedules, and the show basic data page, that contains all mostly
static data for a show and which is usually maintained by
administrators.

refs #181

This is a helpful ADialog wrapper that manages it’s open state by
itself.

Bootstrap only displays form group errors if the .invalid-feedback
element is preceded by a .is-invalid element. That is an uncessary
restriction and we can simply enforce the errors to be displayed if it
is rendered.

Django rest framework would sometimes return objects like this

  { "email": ["Gib eine gültige E-Mail Adresse an."] }

which we didn’t properly handle.