[#46812045] Refactor old config.inc.php

Removed constants related to debugging like this:
- _DEBUG_ - still kept, defined behind the scenes for backwards compatibility; depends on Kernel debug parameter
- _DEBUG_OUPUT - completely removed; the code for writing to firebug is no longer used, we will use monolog handlers for that instead
- _DEBUG_DB_OUTPUT_ - completely removed - will be logged at all times, depending on error level
- _LOG_SLOW_QUERY_ - completely removed - will be enabled always if debug is on; settings in debug module