Context:

In the Backend senate ( notes here ), we recently discussed the instance start and end date filter IT: we seem to have multiple tests specifically testing the behaviour of fixed date filters with undefined start or end, which yields an "after/before" logic, eg "all instances started after 06.09.2021" would be a fixed start date filter with start set to 06.09.2021 and end being null.

In the UI however, we do not allow start and end date filter creation with a `null` start or end. 

Since the BE already seems to have this feature however, we can enable it in the UI as well.

Note: currently we only have IT for the instance date case, though since flowNode date filters use the same filter logic the same should work with flowNodes as well. There is a BE ticket to add some IT for flownode date filters too (OPT-5565).

AT:

• Date Filter dropdown has two more options:
  • after: allows selecting the start date only
  • before: allows selecting the end date only
• Fixed Date option is renamed to 'between'
• The two new options are available in all date filters:
  • Instance start and end date filters
  • Flow node start and end date filters
  • Variable date filters
  • Dashboard start and end date filters