Autosave

Adds autosave for previously created posts
Adds previews for posts
Adds JS alert when leaving a page with unsaved content
you don't need your 2nd check to @post.autosave (the one right after the OR operator). If it gets there, the autosave is non-nil.

```
if @post.autosave.nil? || @post.autosave.content != params[:post][:draft_content]
```