ref https://linear.app/tryghost/issue/ENG-1078
- adds required setup for acceptance testing post revisions
- adds happy-path test for listing, previewing, and restoring a post revision
no issue
- added `page` model
- removed `page` param from Post model
- added pages screen with associated links
- added `:type` param to editor screens to work with the right models
- removed post<->page toggle and associated tour item