Theme preview: Create preview page and show wp-themes.com preview

[ryelle]

See #38 — This adds a new path to the site, ex https://wordpress.org/themes/twentytwentyfour/preview/, which will show a new preview.html template. The preview template sets up 2 columns, on the left is the theme information, and the right has the iframed preview of wp-themes.com.

This matches the current production previewer with the following features:

• The page loads the theme preview, with theme info
• The preview can toggle between three screen sizes
• There is a download button
• There is a back button to return to the theme details page
• It is possible to preview a pattern page /twentytwentytwo/preview/?pattern_name=twentytwentytwo%2Ffooter-about-title-logo
• Viewing a style variation shows the theme with that applied /twentytwentytwo/preview/?style_variation=pink
• New: Combining the two also works /twentytwentytwo/preview/?pattern_name=twentytwentytwo%2Ffooter-about-title-logo&style_variation=pink

Additionally, I've added in the available patterns to the sidebar, clicking a pattern will load that preview. This can create a very long list though, so I can easily remove it if necessary.

In following PRs:

• Adjust the frame on small screens
• The style variations list is not available yet, once the block is built it can be added here
• Maybe remove patterns list

Screenshots

	Screenshot
Previewer (default)
Previewer (mobile)
Previewer (different style variation)