Add Beta Testing page #6269486

Changes proposed in this Pull Request

More info: pd2n8V-46-p2

This adds the Beta Testing page which showcases info about the Horizon programme.
The menu link is added to the Public API via D63456-code and #54588 (in the fallback menu).

Proposed design:

Testing instructions

• Go to the /beta-testing page
• The page should render properly on any screen size

• The Join the Beta Program link should go to horizon.wordpress.com

• The Learn how it works link should go to the About page on horizonfeedback.wordpress.com

• The Recent Beta Features should populate with the last 3 posts from horizonfeedback.wordpress.com

• There should be no related JS errors