Administration and Site Health: Make information on Search engine visibility more prominent.

- makes the "Search engines discouraged" message on the Dashboard more prominent
- adds "Search engine visibility" information under Site Health > Info > WordPress 
- removes title case in favor of sentence case in all the related strings 

Props audrasjb, swissspidy, emanuel_blagonic, Presskopp, karmatosed, valentinbora, melchoyce.
Fixes #35288.

Built from https://develop.svn.wordpress.org/trunk@47859


git-svn-id: https://core.svn.wordpress.org/trunk@47635 1a063a9b-81f0-0310-95a4-ce76da25c4cd