Database query tool should pick up the current database settings of the running wiki

Currently, the query tool of the admin tools allows the user to control the parameters of the database connection (user, password, url) through a form before sending the query.
However, these values are hardcoded to the default mysql settings and thus this tool will not work if the configured database is anything else than mysql with the default settings.

The tool should pick up the settings of the database the wiki is currently connected to and propose those as default values (password should be hidden, though), so that by default the query works on the database to which the wiki is connected.

This will probably fix issues like ADMINTOOL-50 and ADMINTOOL-61 .