feat(ia): cleanup site design

[jaredrethman]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

• Removed Site Design PHP class, SCSS and Javascript.
• Updated link in components demo.
• Reused Newspack -> Settings -> Theme and Brand component in setup.
• Fixed PHP test to use a permanent Wizard endpoint.

How to test the changes in this Pull Request:

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?