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.
Another thought: it is important to test the UI when the server responds normally, but then also test what happens when the server responds with an error. It is easy to simulate this with the create_sinon methods ```respondWithJson``` and ```respondWithError```.