[1.6.x] Reworked the detection of local storages for the collectstatic command.

Before 4befb301 the detection was broken because we used isinstance
against a LazyObject rather than against a Storage class. That commit
fixed it by looking directly at the object wrapped by LazyObject.
This could however be a problem to anyone who subclasses the
collectstatic management Command and directly supplies a Storage class.

Refs #21581.

Backport of 7e27885c from master.