BLD-1104: List units that use group configuration.

**Ticket:** [BLD-1104](https://openedx.atlassian.net/browse/BLD-1104)

**Acceptance criteria:** 

* In collapsed view of group configurations: 
 * Show number of units that use this group configuration.
* In expanded view of group configurations: 
 * Show list of units that contain experiments that use the group configuration
 * Each item in the list of units should be hyperlinked. When the link is clicked, the unit containing the experiment that uses the group configuration should be opened. 
 * If no content experiments use this group configuration, show link to Course Outline to use the group configuration. 

**sandbox:** http://studio.jmclaus.m.sandbox.edx.org/

@andy-armstrong , @cahrens , @frrrances , @mhoeber , @explorerleslie please review.
@cahrens ```This is just a single usage, correct?```

Yes, so I changed the var name to ```usage```.