refs https://github.com/TryGhost/Team/issues/559
refs https://github.com/TryGhost/Team/issues/1277
- switched modal implementation to the newer promise-modal style
- added `<GhEmailPreviewLink>` component that renders a link that when clicked opens the modal
- removes the need for templates/controllers to manually handle modal opening/closing and to pass actions down from parents
- updated all places we were triggering an email preview modal to use `<GhEmailPreviewLink>`
