DAG MVC: Check feature flag explicitly

What does this MR do?

A follow-up to the bugfix in !32966 (merged), this MR explicitly checks for the feature flag before loading the DAG app.

We still need to add more tests for the pipeline page as a whole — the issue for that is here: #219024.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.