Editor, Preview: Preview obscured by masterbar and undismissable, iPhone 6+

The preview modal is undismissable on my iPhone 6+. Preview loads beneath the masterbar resulting in the dismiss button being obscured. Escaping the modal can mean abandoning editor changes and later restoring them.

Related: https://github.com/Automattic/wp-calypso/issues/2566

#3339-gh-wp-calypso, #editor, #ios, #phablet, #preview