Ember editor save and publish button functionality

Closes #2747
- Added new 'editor-save-button' view and template.
- Added save action to post controller.
- Set a new post as the default model for the /editor/ route.
- Set the `posts/post` controller as the controller for the /editor/
  route.
- Needs ghost-popover #2418 component for full pop-up functionality
Merge pull request #2786 from novaugust/ember-editor-save-and-publish

Ember editor save and publish button functionality