DAG MVC: Interactions

What does this MR do? + Screenshots

This MR adds the interactions for the DAG graph as referenced in #215517 (closed). Please note: everything remains behind a feature flag.

These are:

interaction	screenshot (note the blocky gradient is an artifact)
On link hover, the pointer cursor (the little hand) and the link and its related nodes are highlighted; this highlight disappears when the link is no longer hovered.
On link click, the link and its related nodes are highlighted and the highlight persists until the link is clicked again.
On node hover, the pointer cursor appears.
On node click, the entire path prior to the selected node is highlighted. This view is toggled on subsequent clicks.
In combined cases, the graph will remain in "background mode," with unselected items de-emphasized until no highlights are applied.

Local Replication

The sample gitlab-ci.yml files in #215524 (comment 341935827) can be added to a projects on your local instance to create test graphs.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

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