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.
@cahrens 
> It is better to put the CSS into the page object. So the page object would have methods to return whether or not an icon is present and whether or not the message text is shown.

You are quite right, it is done for the tests added in this PR. I also moved the CSS out of another test, ```test_no_group_configurations_added```