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/
I know that this was copied from textbooks.html, but it doesn't seem like this sidebar is very reusable. I'd rather see a shared component that renders all these links in a standard way, and that is smart enough to drop the current page from the list. We could then use this shared component on every settings page. In fact, we might want to include the current page in the list as well, but just make it clear through styling that it is the current one and show isn't linked.