Implement safe_exec on top of jailpy (old unsafe safe_exec is still here); Remove some crazy stuff from the context; always pass globals and locals, locals are the things that can be changed.

fix broken test. Since we're pre-populating the course overview when creating a course, the clone_course fails because it asserts that the destination course that it clones into is empty. Need to check that the course is basically empty (e.g. a course module and a overview module)

add a delete-all-versions flag to the delete_item method in the modulestores. Extend tests to verify. Still wip