add display_coursenumber and display_organization fields on the CourseModule

Review notes:

This PR decouples the display of ORG/COURSE from the underlying URL/Location scheme which has some 'legal character' constraints. This will be needed for these two use cases in the near term:

a) We have international course authors that may wish to use non ascii characters in their ORG/COURSE pairs. 

b) We want to be able to allow authors to override the display behavior, particular vis-a-vis course-reruns which may require unique ORG/COURSE pairs but authors want the display of the re-run to be something else than exposing the raw underlying URL/Location-based information

@cpennington @dmitchell @ormsbee Can you review? 

Existing tests pass locally. I need to build out some more unit tests, but I wanted to get the review process started.

NOTE: I wasn't sure what we should do regarding sorting behavior, so I left that alone.