Simplify the setup-config.php UI flow and load process.

When no configuration file is detected, we now redirect to setup-config.php. This process now uses the WordPress bootstrap, rather than a set of fragile hacks.

fixes #28740.

Built from https://develop.svn.wordpress.org/trunk@28978


git-svn-id: http://core.svn.wordpress.org/trunk@28767 1a063a9b-81f0-0310-95a4-ce76da25c4cd