general: legacy directory pre-creation

* Ensures legacy directories are pre-created
  upon legacy extension setup_app() call.
  (closes #1789)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
I find it lazier and nicer, but that's your call. I wouldn't worry about bloating the logs though, eventually sentry will have your back on that.