Fix editor markdown tab

Fixes #3813, reference #3810

* Add showPreview action to controller
* Implement action and bind attribute
* Remove the now pointless

![test](https://cloud.githubusercontent.com/assets/4590364/3977943/f52f1004-2840-11e4-9d72-27f4e3e18473.gif)


You can group all these classes together like so:

```hbs
<section {{bind-attr class="isPreview::active :entry-markdown :active"}}>
```
Would you mind cleaning this and the other section up like that? :D