Move WordPress & Gutenberg PR Preview to Playground website

[ajotka]

Motivation for the change, related issues

https://github.com/WordPress/wordpress-playground/issues/1655#issuecomment-2371823139

I moved wordpress.html and gutenberg.html to Playground itself. By the way, I also made a mini appearance refactor for the modal, based on Figma. And I used components from WordPress packages.

Implementation details

• [x] Add modals with input to fill PR number
• [x] Move & unify logic to fetch PR by number
• [x] Error handling
• [x] Appearance refactor for Modals
• [x] Implement new Modal to other components
• [x] Fetch PR number directly from query
• [x] New Dropdown Menu in main sidebar (thanks to this, we can import playground when we don't have any active one)

Testing Instructions (or ideally a Blueprint)

• Any blueprint
• Go to Playground settings
• Click three dots next to ~~Homepage button~~ future Logo place
• Select Preview a WordPress PR or Preview a Gutenberg PR

It is also possible to open modal via URL params: /?modal=preview-pr-wordpress or /?modal=preview-pr-gutenberg