DAG Visualization MVC - feature flag

What does this MR do?

Adds a feature flag that will be used for the new DAG visualization in the pipeline page. If the feature flag dag_pipeline_tab is on, then we show an additional tab in which we will only show the DAG jobs relationship. This MR also adds the empty JS file that we will use to instantiate the graph.

Screenshots

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After:

Does this MR meet the acceptance criteria?

Conformity

• Documentation (if required)
• Code review guidelines
• Style guides