Add a new class called addgatewaybox to make it easier to respect custom themes. Fixes #2900

Conflicts:
	usr/local/www/interfaces.php
	usr/local/www/themes/_corporate/all.css
	usr/local/www/themes/code-red/all.css
	usr/local/www/themes/pfsense-dropdown/all.css
	usr/local/www/themes/pfsense/all.css
	usr/local/www/themes/pfsense_ng/all.css
	usr/local/www/themes/the_wall/all.css