[UX] Provide indication in the admin interface, when config values have been overridden in settings.php

Description of the need

Now that config values can be overridden via settings.php, we've created a system that can also be very confusing for admins and editors. When a value is overridden it appears as though the forms are broken.

For example, when CSS aggregation is set to FALSE in settings.php, visiting the form at admin/config/development/performance appears as usual, but when I attempt to check the box for "Aggregate and compress CSS files" and submit the form, it submits as usual, and the message The configuration options have been saved. is printed to the screen. However, when I scroll down to the checkbox for "Aggregate and compress CSS files" it remains unchecked.

Proposed solution

@herbdool posted a proof of concept PR that contained some interface components that indicated when a config value was overridden. I'm not sure how far this proof of concept is from a workable solution, but I really liked where it was heading:

Additional information

This is a follow up to https://github.com/backdrop/backdrop-issues/issues/2805