BLD-1125: Validate content experiments after managing a group.

As a course author, I want to validate my content experiments after managing a group.

**Ticket: [BLD-1125](https://openedx.atlassian.net/browse/BLD-1125)**

**Acceptance criteria:**
* On save of a group configuration, Studio should validate all of the content experiments in the course that use it. If there are any messages then they should be displayed below the unit name/link.
* In addition, whenever a group configuration is shown, its content experiments should be validated, and any messages shown in the same way.

**Sandbox:** http://studio.olmar.m.sandbox.edx.org/

@explorerleslie @frrrances 

@tymofij @cahrens  please review.
You could create a helper method to:

1. create the user_partitions for the course (perhaps passing in the Groups, since that bit does differ slightly, though I don't think it actually needs to)
2. Assign a vertical to use the first group configuration
3. Go to the group config page and edit the first configuration?

The same code appears in 3 of the test cases. I realize that this may be stylistic and perhaps you prefer each test case to be very explicit about what it is doing.