Create new group configurations page.

**Ticket**: [BLD-1121](https://edx-wiki.atlassian.net/browse/BLD-1121)

**Acceptance criteria:**
* Button (or something) on the Group Configurations page to create a new configuration. 
* Editable fields appear for: 
  * Group Configuration Name (required, ok if non-unique)
  * Group Configuration ID (READ-ONLY, required, numeric, must be unique within a course - can just increment sequentially for courses with no existing but take into account courses with existing group configurations created in the user_partitions value on Advanced Settings page - we must not overwrite any values.)
  * Group Configuration Description (optional)
* DO NOT show any Groups information on the front end (will be a separate story) but DO create default groups (2 groups, named Group A & Group B, both at 50% allocation) on the backend. (This should already be handled by the split_test module, but we should test to make sure this reflects correctly in the Group Configurations page.)
* Create button should save the group configuration.
  * Error message should appear if required fields are omitted upon save. 
  * User should stay in the New/Edit Experiment modal to correct the error. 
* Cancel link should not save but should close the new group configuration modal. 

**sandbox:** http://studio.olmar.m.sandbox.edx.org/

@andy-armstrong , @cahrens , @frrrances please review.
I don't like returning HTML for 'not JSON', and then returning JSON for everything else. I'd rather have explicit tests for HTML and JSON and throw a 406 for everything else.