refs https://github.com/TryGhost/Team/issues/998
- posts have a new `email_only` property for an alpha feature but that is not a valid property for pages
- our pages model and serializers currently inherit from the post model/serializer so when saving the new property is triggering a "no additional properties" validation error
- updating the pages serializer to remove the new property means the pages add/edit API requests are valid again
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
