- Permit a nil navigation controller if we're in a preview platter
- Instead of directly pushing onto the navigation controller's stack,
ask the split view controller to present the thread when we pop.
- Instead of hiding the input toolbar when shown in a preview platter,
set the bottomViewType to .none. Content insets are determined by the
bottom bar size. Even if the input toolbar is hidden, we were still
using its height in our content inset. This fixes that.
