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 
>I would split this into 2 scenarios (2 different test points). The first is the case of no usages showing link to outline page. The second is the case with a usage.

I've split the above test into two tests, named: ```test_group_configuration_empty_usage``` and ```test_group_configuration_non_empty_usage```