[DX] Add support for skip_permissions_hardening in settings.php

[klonos]

This is the respective ticket for https://www.drupal.org/project/drupal/issues/1232572 and part of #378

It's common for sites in version control to include a settings.inc file in sites/default that is used to version control site configuration, while keeping sensitive information (keys, passwords, etc) in settings.php. When system_requirements() runs, it will automatically change the permissions of the site directory to remove write permissions. Then, when running git pull, the merge will break because write permissions are missing on the directory.

The skip_permissions_hardening variable can be used to disable chmod'ing files. system_requirements() still throws an error, but I think that's desirable. I'd be open to downgrading the requirement to a warning if this variable is set.

If this approach is sound, we'd still need to add an example to settings.php so it's known that the variable exists.

This has already been implemented in D8, and that ticket is now set to be backported to D7, but similarly to #3459, the way things are progressing for D7, it might be another year+ before we see this being implemented. No reason we can't do it sooner in Backdrop 😉