Users should not be able to delete custom segments created by Super User

[mattab]

It was noted that normal view/admin users, can delete segments created (and shared with them) by the Super User. Instead, we should prevent such "super user segments" to be deleted by view/admin users.

[mattab]

In 424f3d7: Fixes #4580 - Make sure users editing / deleting segments have enough permission to do so (either Super User, or the user who created the segment)