Hide subtasks' Fix Versions without interfering with Work Sub-filter

According to JRA-5225, subtasks do not inherit Fix Versions of parents when Fix Versions are updated, and vice-versa. Because of this, sometimes parents have Fix Versions but subtasks do not (created without Fix Versions). This causes confusion to users who work on subtasks, and there's a chance that they will update subtasks with Fix Versions which do not follow parents'.

Suggestion is to be able to easily hide subtasks' Fix Versions in Agile boards. Currently this can be done by using Field Configurations, but once the field is hidden, subtasks no longer appear in Kanban boards as the default Work Sub-filter will filter them out - Hidden is not treated as Empty.

The only feasible workaround is to manually adjust the Work Sub-filter to include subtasks.