Allow appending of obs_ids to existing group

The create/update group screen now has a field "Append SAS ids". These will always be appended to the group:

Some edge cases were fixed:

• Propagating filters from group to workspecifications did not work when a previously set filter was removed
• After editing filters on a group (and thus work specifications), inputs will be updated on existing tasks
  • Edge case: if tasks are already submitted, group changes should not be propagated to them