Split up config.php to clarify use

[prodigion]

Currently, config.php has configuration parameters, reads configuration from an environment file, validates configuration, and has sql transaction code. These should be mostly split to separate files to separate what users should/shouldn't touch.

Default configuration could go in a separate file (/includes/DefaultSettings.php), with site-specific configuration could go in its own file (LocalSettings.php). Or put it all in the database. It shouldn't rely on environment variables or .env files

Any logic should be moved to a separate file.