@gib: Add a slideshow section for migrated posts

To accommodate migrated posts we're going to add a slideshow UI at the top of an article. This will contain artworks, video, and images in the order they were added to the post.

Right now this just adds slideshows via the migration script and only gives the option to remove the entire slideshow section. Down the line we'll add the capability to edit and add slideshows.

![cap](https://cloud.githubusercontent.com/assets/555859/5099086/56388ad4-6f5d-11e4-850a-7f6843e944b0.gif)