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/

**Note**:  Bok-choy tests left to be done.

@explorerleslie @frrrances 
```python
validation_messages = split_test.validation_messages()
has_error = any(message.message_type == 'error' for message in validation_messages)
if validation_messages:
    return {
        'message': _('This content experiment has issues that affect content visibility.'), 
        'type': 'error' if has_error else 'warning',
    }
```
`None` is returned by default.