Admin: Introduce the Site Health screens.

The Site Health tool serves two purposes:
- Provide site owners with information to improve the performance, reliability, and security of their site.
- Collect comprehensive debug information about the site.

By encouraging site owners to maintain their site and adhere to modern best practices, we ultimately improve the software hygeine of both the WordPress ecosystem, and the open internet as a whole.

Props Clorith, hedgefield, melchoyce, xkon, karmatosed, jordesign, earnjam, ianbelanger, wpscholar, desrosj, pedromendonca, peterbooker, jcastaneda, garyj, soean, pento, timothyblynjacobs, zodiac1978, dgroddick, garrett-eclipse, netweb, tobifjellner, pixolin, afercia, joedolson, birgire.
See #46573.

Built from https://develop.svn.wordpress.org/trunk@44986


git-svn-id: http://core.svn.wordpress.org/trunk@44817 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/wp-admin.css

@@ -12,3 +12,4 @@
 @import url(widgets.css);
 @import url(site-icon.css);
 @import url(l10n.css);
+@import url(site-health.css);