Fix HTML preview rendering on the main screen

closes #2904
- The `posts/post` template now takes its HTML from the post model instead of using the editor's markdown component
Merge pull request #2905 from appleYaks/preview-markdown

Fix HTML preview rendering on the main screen
Split PostController amongst new Editor/PostSettingsMenu Controllers

closes #2910
- create EditorController, PostSettingsMenuController
- remove `posts.post` controllerName dependency on EditorRoute/NewRoute
- `{{render}}` the PostSettingsMenuView with its own controller
- break up properties/methods from PostsPostController into all three controllers
- fix EditorRoute pagination options to now be comparable to PostsRoute
- add NewController to force NewRoute to refresh editor with blank model. Identical to Editor's
- EditorController and NewController are based on a shared mixin
- NewView created, templateName is 'editor'