Group Configuration: create, edit, delete groups

**Ticket:** [BLD-1110](https://openedx.atlassian.net/browse/BLD-1110)

**Acceptance criteria:**
* Two Groups by default:
  * Both with 50% allocation
  * Default names Group A and Group B.
* Adding another group should reduce the allocations of the existing groups equally, so that all groups are equally allocated. (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.)
  * Default name of new groups should follow alphabetically: Group C, Group D, etc. 
* Group names should be editable. 
* Deleting a group should remove the group, reallocate the percentages equally between the existing groups. 

**sandbox:** http://polesye.m.sandbox.edx.org/ 

@andy-armstrong , @cahrens , @frrrances , @mhoeber , @explorerleslie please review.
> Why this assignment? Just to make it clear that it is an underscore library method?

It's convenient to know that we're using underscore.string. But from another point of view it looks redundant. I'll remove it.