a8n: Fix Changeset syncing by not marking newly created changesets as deleted

Created by: mrnugget

The problem: we always wrote a value to the nullable external_deleted_at column on changesets. But the syncer that updates the changesets only syncs changesets that have not been marked as deleted. That means: every newly created changeset since the introducing of the external_deleted_at column has not been synced.

The fix: do not write external_deleted_at if the value has not been explicitly set. Add a migration that resets all changesets that have been marked as deleted to not deleted.

This is on me. An oversight that I covered with the wrong assertion in the tests. It only popped up after I noticed that changesets weren't synced.

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The time.Time{} value got translated to 0001-12-31 16:07:02-07:52:58 BC in my database. Does anyone know whether that is stable across timezone configurations on machines? If so, I'd maybe extend the migration to only reset the column WHERE external_deleted_at = '0001-12-31 16:07:02-07:52:58 BC'. But that's not strictly necessary since even if we reset all deleted timestamps, the next sync run will set them again.