More widgets, previewed on phone

The widget editor's phone preview, added so merchants could see edits the way a shopper on a phone actually would, previously covered only some widget types. Four more now open on the same phone-sized preview.

Build a box, the membership Access denied page, Fixed bundle, and Fixed bundle Included products all draw now with the storefront's own phone layout when a merchant edits them, the same preview already available for other widgets.

This change touches only the admin editor's preview. It doesn't change any shop's live subscriptions, offers, billing, or storefront widget, a merchant editing one of these four widget types simply sees a more accurate picture of how it will look on a phone while they work.