Merge in changes from edx-platform-master branch.

the number of processes and threads to allow.

Update docs with arch diagrams.

Update README.rst

Add an OEP-2 compliant openedx.yaml file

Fixing django integration.

Restructure codejail to allow more flexible jailing

Jails provide a broader configuration for a given command.  Language
provides programming-language configuration, so multiple jails can exist
for a given language without duplicating configuration.

* jail_code and safe_exec now live on the the Jail object
* codejail does not autoconfigure an insecure codejail at import time
* safe_exec can now be run for python3 codejails

Fix command line handling so it works when no user is specified.

Optionally use a proxy process to spawn subprocesses

When spawning subprocesses, fork() will fail if the memory of the parent
can't be duplicated in the child. This means large long-lived processes
will have problems using CodeJail repeatedly.

This introduces a long-lived proxy subprocess which then spawns the
actual subprocesses.  Because the proxy process starts small and stays
small, it will be able to spawn subprocesses even when the parent
process grows large.

The use of the proxy process is controlled by a "PROXY" limit, which
should be 0 or 1.  If neither is set, then the CODEJAIL_PROXY
environment variable determines whether the proxy is used.

Change from AGPL to Apache

Set CPU limit to get better info. More specific tests

Fix a test to allow for the exception to be repr'd.

add more logging for TNL-835

Improved error message with status code on unsuccessful subprocess.

listdir is not gauranteed to be sorted.

Feanil/full temp path

Importable zip files

extra_files can now include a zip file that can be also indicated in
python_path, and it will be importable by the executed code.

typo fix in Readme

Clean up CodeJail documentation