Consider using storybook for visual regression testing of the primary navigation dropdown menu

The following discussion from !101120 (merged) should be addressed:

• @lmeckley started a discussion: (+1 comment)

  Thought (non-blocking): Instead of unit testing the UI classes, this would be a great opportunity to use UI regression testing to verify the placement and colors of the UI. Gitlab-UI leverages the Storyshot framework for this. That's out-of-scope for this MR, but would better validate the behavior the unit tests are verifying.