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>
```

I made it work with static HTML content only for performance reasons (data attributes are searched once at load and not on every click, so it wont work with generated html)

As discussed on #owncloud-design

@jancborchardt @LEDfan @MorrisJobke @Kondou-ger @jbtbnl