Converter: Multiple converter improvements

• Move classes into their own files
• Move converter.php into /admin/tools/
• Escape output of translated strings where possible
• Introduce bbp_converter_db_connection_failed error handler, in the event the origin database connection cannot be established
• Sets WP_SETUP_CONFIG constant to true to control $wpdb->db_connect() bail from using db-error.php
• Use sanitize_text_field() on database connection values
• Invisibly default to DB_ constants if database connection values are empty
• introduce convert_users class parameter to limit direct touches to $_POST value
• Smarter defaults for converter options