closes https://github.com/TryGhost/Ghost/issues/12420
- adds "Open rate" to the available ordering options in the posts list order dropdown
- amends pages controller to have it's own order list because pages can't be ordered by open rate
no issue
- posts and pages (and their corresponding `-loading` counterparts) templates were all duplicating a lengthy section of handlebars
- extracted the duplicated handlebars into a template for easier design changes or upgrading of `ember-power-select`
- moved generation of the "selected" CSS classes into the template to clean up code in the controller
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
