message: fix: 'Save Chart' modal's dashboard dropdown isn't sticky
  It appears that the logic that makes the dashboard selection sticky
  in the 'Save Chart' modal (explore) was broken at some time.
  While working on a dashboard, it's nice if this modal can remember the
  dashboard selection.
diff: @@ -873,6 +873,7 @@ class Superset(BaseSupersetView): # pylint: disable=too-many-public-methods
  "form_data": slc.form_data,
  "slice": slc.data,
  "dashboard_url": dash.url if dash else None,
  + "dashboard_id": dash.id if dash else None,
  }
  if dash and request.args.get("goto_dash") == "true":
  