refactor: use django_filters FilterSet for APITimeSlotViewSet

This change re-implements all existing collection filters for the
APITimeSlotViewSet with a FilterSet. No breaking changes are expected,
though there are some changes in semantics:

* The start and end query parameters no longer need to be specified
  together. If users only want to modify the start or end date they
  can now do that.
  If start is specified and end is not, end will be start + 60 days.
* If end was not set it would default to start + 60 days at 00:00.
  This is now fixed and end will be start + 60 days at 23:59:59.
* end now uses time.max, which selects the latest possible time on
  the specified date.
* The surrounding-filter now accepts a datetime value and will be set
  to the current time if only the query parameter but no value
  has been set.
* All filters are now applied in series. This wasn’t the case for
  every filter, e.g. the surrounding-filter would return early.