Site Health: Introduce Site Health Status dashboard widget.

The widget informs administrators of any potential issues that should be addressed to improve the performance or security of their website, and directs them to the Site Health screen for more details.

Props Clorith, hedgefield, guddu1315.
See #47606.
Built from https://develop.svn.wordpress.org/trunk@47063


git-svn-id: https://core.svn.wordpress.org/trunk@46863 1a063a9b-81f0-0310-95a4-ce76da25c4cd