Added slightly broken support for allowing user and admin customisation for turning on and off the AJAX page exit warning. Unsure of best way to debug with this codebase, but there is failure at the saving part for users/admin editing when saving settings, it seems to silently fail or simply do nothing. Otherwise the feature appears to be working fine.

New appropriate language vars added for the feature, Japanese translation also provided. Installation script and update script also provided for new database column in user and admin tables