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.
> Did you consider getting this message from the split_test instance itself (similar to how we get the validation_messages from the split_test instance)? That would guarantee that the message stays in sync between the rendering on the container page and the configuration page. The message would move out of split_test_author_view.html and into split_test_module.py.

Added `general_validation_message` to `SplitTestDescriptor`.