refs #242

refs #181

refs #246

refs #206

refs #206

fixes #169

Apparently there were still usages of the $log object we’ve removed in
52227b1d. We fix that by simply using
window.console.

* can now be used to represent date, time and datetime strings
* does support nullish-values with proper types
  (and adds a corrosponding default value)
* allows timezone offset to be stripped for use
  in datetime-local input fields

refs #177

fixes #100

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)