fix(button-menu): ensure button menu closes when tabbed away (#549)

`this.mq` is a string, `this.mql` is a `MediaQueryList` which has a matches method, as was
originally intended

src/moj/components/button-menu/button-menu.js

@@ -124,13 +124,13 @@ MOJFrontend.ButtonMenu.prototype.onButtonKeydown = function(e) {
 			this.focusNext(e.currentTarget);
 			break;
 		case this.keys.esc:
-			if(!this.mq.matches) {
+			if(!this.mql.matches) {
 				this.menuButton.focus();
 				this.hideMenu();
 			}
 			break;
 		case this.keys.tab:
-			if(!this.mq.matches) {
+			if(!this.mql.matches) {
 				this.hideMenu();
 			}
 	}
@@ -152,4 +152,4 @@ MOJFrontend.ButtonMenu.prototype.focusPrevious = function(currentButton) {
 	} else {
 		this.container.find('[role=menuitem]').last().focus();
 	}
-};
+};