Edx-codejail

This release adds a single logging statement for every call to `jail_code`, including the call's slug and effective resource limits.

Developers: This release also includes a variety of code cleanup. It adds the ability to check code quality/style via `make quality` or `tox -e quality`.

Adds a `limit_overrides_context` argument to `jail_code` and `safe_exec`.

When provided, if `settings.CODE_JAIL['limit_overrides'][limit_overrides_context]` exists, then prefer those values to the ones in `settings.CODE_JAIL['limits']`.

This allows limits to be altered for specific contexts (for example, in the context of a certain course run, the permitted execution time could be increased).

We removed support for detecting older virtualenvs but then realized that edx-platform deploys on a version of ubuntu that still user the older version of the virtualenv library.