refs #102

This adds proper labels and description for what is happening.

fixes #193

fixes #235

Up until now the dashboard used a hardcoded list of rrules that matched
a long-gone fixture set of rrules in steering. Additionally, rrules were
translated in the dashboard using the the ID of the rrules in steering.
This means that any change to the fixture set, removing an rrule or not
importing the fixture set at all would mix up the translations and
therefore the actual schedules that people would create.

A few months ago the rrules were reworked and an API endpoint was
introduced that exposes the rrules defined in steering. No appropriate
change was made in the dashboard and so the rrule definitions got
out-of-sync.

This change implements a rrule store that fetches data from the API.
Additionally we’ve also implemented a custom rrule to natural language
mechanism [1]. This is because the scope of the rulesets we use in the
AURA radio context is well-known and not to complex and because rrule.js
built-in `toText` function, which might have been an option, is limited:

* bysetpos seems to be entirely unsupported
* internationalization will produce gramatically questionable
  output for languages like German, because the translation
  interface is somewhat lacking.

[1] #226 (comment 13669)

fixes #217

fixes #189

fixes #215

undefined duration has special handling in the playout so this should be
supported.

fixes #211

fixes #198

fixes #212

refs #127

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.

fixes #199