Enable DEFAULT_MODE to be customized in config
Keeps platform set default of "Audit" to be backwards compatible and not break stuff
This should be configurable because many installations outside of edX give free completion statements, and the default "Audit" mode does not allow for that...
@peter-fogg I'm guessing you'd be a good person to look at this since you converted the default mode to Audit: https://github.com/edx/edx-platform/commit/497622d4b44049b5f0260347c3b1d3c56958461c
I moved this to ECOM, as they have done all the course mode code.
While this is a T&L feature, it might also make sense to ask for to be a second reviewer perhaps on this work since he was maybe the last person to review code relating to course mode (though Bill if this is something that Peter largely handled you can safely say "not it" for even a secondary reviewer).
Next step would be T&L team assignment.