Add mixed PostgreSQL config.

This PR adds new PostgreSQL config called "mixed" that tries to solve issues with standard and custom configs:
* most values in the standard config are commented out and therefore PostgreSQL ignores them. TBH I have no idea how standard config is supposed to be used.
* custom config does not include default values which breaks tools like pg_wrapper, because pg_wrapper tries to read `data_directory` setting from the postgresql.conf file. It also breaks default logging settings.

PS Ideally I would like to remove standard and custom config templates and keep only mixed config template.