BLD-1117: Add read-only list of Group Configurations.

Create group configuration page framework:

  * If no group configurations in course:
    - Show Group Configurations page with no group configurations listed. (No "Create new group configuration" button yet.)
  * If group configurations exist in course:
    - Show, for each group configuration, show the group configuration name and number of groups.
    - DON’T show in what units the group configuration is used yet. That will be a separate story.
    - Expand to see description, group names and allocations.
      + Again, don’t show where the group configurations are used yet; this will be a separate story.
    - No “Edit” link at this point (subsequent story will implement).

Configure when the Group Configurations page is visible in the Settings menu:

  *  Before the `split_test` advanced module is enabled, Group Configurations page shouldn’t be visible in the navigation (in the menu bar).
  *  After the `split_test` advanced module is enabled, Group Configurations page should be visible in the navigation.
  *  We know that this will mean that the Group Configuration page navigation will not appear immediately (will appear on page refresh or otherwise navigating to another page). This is a wider problem with advanced settings and will be dealt with in a separate story.

Please review: @andy-armstrong , @frrrances , @mhoeber , @auraz 

Sandbox: http://studio.tymofij.m.sandbox.edx.org/
> This probably needs a translator comment to explain the two strings. It is also interesting that the translation is done immediately rather than in the template.

Done in df2292f