Change the XBlock target hash, not because we need new code, but because the old one was tangled up in a eponymous branch that was causing trouble.

adding pre-requirements for sandbox-requirements.txt

Fix UnicodeEncodeError when generating cache keys

from non-ASCII unicode code submissions.

A new boolean on XModuleSystem that determines whether to allow execution of untrusted unsandboxed code.

Python should have -E, not sure of a clean way to do it, but this at least only applies it to python.