Move Sage 10 to Laravel Mix

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Changelog

Enhancements

• Change to Laravel Mix.
• Clean up and refactor starter styles.
• Bump Bootstrap and related dependencies (Popper.js).
• Add missing trailing space to ! in the body_class filter.
• Use asset()->uri() where applicable for sanity purposes (despite asset() defaulting to ->uri()).
• Add sanity linebreak to the li in comments.blade.php.
• Rename main.css and main.js to app.css and app.js.
• Make app.js comments uniform.
• Add missing finalize() to about.js.
• Change add_editor_style() to app.css as admin.css does not exist.
• Remove autoload from styles/scripts.
• Remove $ import on customizer.js as Mix autoload will handle that.
• Remove sage-installer (will be replaced at a later date).
• Remove unnecessary fields in package.json as it will never be published.
• Bump Node.js version in README.
• Bump dependencies.
• Automatically remove Blade cache when theme is activated, changed, or removed.

Bugfixes

• Fix conditional on comments.blade.php.

Notes

• Sage will no longer lint on build by default. Documentation will be provided to add this yourself. yarn run lint is still available as well as default rules for editor plugins.
• Sage will no longer optimize images on build by default. Documentation will be provided to add this yourself.

Todo

• Change Acorn's manifest/asset class to handle leading slashes. Perhaps Str::start($path, '/')?
• Update documentation for Sage 10.
• Bump Add wp_body_open hook (WP 5.2+) #2167 for when 5.2 releases.
• Sage CLI.

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Have to go out for a few, will fix conflicts when I return. (Sorry)