Create reusable data attribute for apps setting slideup

Provides a way to slide down a target area through a button and slide it up if the user clicks somewhere else. Used for the news app settings and new add new form
 
**Usage**:
```html
<button data-apps-slide-toggle="#app-settings-content">slide</button>
<div id="app-settings-content" class="hidden">I'm sliding up</div>
```
As discussed on #owncloud-design

@jancborchardt @LEDfan @MorrisJobke @Kondou-ger @jbtbnl