The review request tabs element was also adding the review-request-actions

class, picking up a whole bunch of style rules. Most of these rules were

not necessary, either being specific to actions, or overridden by the

tabs.
This change splits those up, and also redoes the tab layout using our

new CSS component style. With this, the layout and CSS use our modern

standards, and use the more-semantic <menu> element. I've also added

ARIA attributes where appropriate to ensure that these elements include

the proper roles and attributes.

Looked at and interacted with reviews, diff, and file attachment tabs in

  Firefox, Chrome, and Safari. Verified that functionality and

  appearance are correct.
Used a11y debug tools to validate the layout and roles of the tabs.