moment is still needed as peer dependency but we should’t use it in our
own code anymore so we can remove it in the future.

The steering API returns HTTP 400 errors for general payload and 409
errors for scheduling conflicts that we didn’t handle yet.

fixes #93

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.