refs https://github.com/TryGhost/Team/issues/1621
reqs https://github.com/TryGhost/Ghost/pull/14820
We want to allow previewing emails based on the newsletter selected in the publish flow.
- passed the selected newsletter from `<PublishManagement>` through to the preview modal then through to the email preview component
- removed tracked `newsletter` property and fetching of the default newsletter in the email preview component that was in place to allow `senderName` and `senderEmail` to be used in the component
- added `newsletter` getter that uses the `post.newsletter` or the passed in selected newsletter
- updated the email preview fetching to pass `?newsletter=slug` so the email is rendered with the correct newlsetter
refs https://github.com/TryGhost/Team/issues/1621
- copied existing preview modal over to `editor-labs/modals` directory
- old modal will be deleted in cleanup
- moved "Preview" button from editor template to the `<PublishManagement>` component
- allows for preview modal to be controlled alongside the publish flow modal
- added `togglePreviewPublish()` action to `<PublishManagement>`
- opens whichever of preview/publish is not currently open, this opens the new modal on top of the old modal
- waits for the modal animation duration to pass then closes the modal that's now underneath, this prevents the flashing that occurs when modals are both opening and closing at the same time because that results in a 50% opacity of both modals during the middle of the animation
- updated preview modal and publish-flow modals to have "Publish" and "Preview" buttons respectively that call the `togglePreviewPublish` action
- updated preview modal to be fullscreen to better match the publish modal
