Stg plg system debug log category mode

Like #2253 but for staging instead of master.

Basically, since `JLog` now supports category exclusion instead of just inclusion, the logging feature in this plugin should also give that option. Exclusion is much more useful than inclusion in this case anyway because you probably want to see most logs but you will never want to see `databasequery` logs and you might get tired of looking at `deprecated` logs.