refs #5652
- add `ember-resize` addon that registers a single resize event handler and exposes it as a service and mixin
- add a component that wraps the posts list and content preview and exposes a `previewIsHidden` property
- use the `previewIsHidden` property in `gh-posts-list-item` to switch the item's link between the editor and the preview
- add `display: none` to the preview pane when in mobile so that we can test it's visibility
