Import epics alongside with its epic work item

If we need to maintain the import/export of Epics from older exports we can lean on existing backfilling migrations code, move it to some PostImportSyncWorkItemsService and call that at the end of the import.

The idea being that we import Epics as we currently do, and then we basically run a backfilling migration on all Epics in that specific group and it's child groups.

Notes:

• This will be possible to the moment when we want to start slimming down the epics table itself, then we need to get back to the same discussion of restricting epics import at least to some degree
• This will also prevent us adding some data integrity constraints sooner rather than later. E.g. we cannot add a NOT NULL constraint to epics.issue_id because we have to create the epic first and then run the backfilling, adding the constraint would just fail to create the epic in first place, thus there will be nothing to backfill afterwards.