Use the private filesystem for config directories, and expose a private filesystem path setting in the installer

Related:
#1444620: Remove file signing from configuration system
#1914018: hook_requirements() for un-proteced configuration directories
#1183044: Test whether htaccess protection works

Goal

• Decrease security checks and validations to a single point of failure.

Proposed solution

1. Change the installer to expose a required setting for the private filesystem path.
2. Store the private filesystem path in settings.php.
3. Create config directories in the private filesystem path.