Mobile sidebar swipe

APP DEVS: If tapping on an element in the app-navigation switches to the content and you don’t want that (for example if it’s a button or so), use ```.app-navigation-noclose``` on the highest element in which you do not want taps to close the navigation.

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Fixing https://github.com/owncloud/core/issues/176

This will make ownCloud usable on mobile again since we introduced sidebars. Specifically this is about having a way to toggle between the sidebar (app-navigation) and content (app-content).

This is done using [Snap.js](https://github.com/jakiestfu/Snap.js/) and works via swipe as well as with the button in the top left.

TODO:
* [x] settle on a way to do the sidebar – FIXED, YEAH! Thanks to everyone! :)
* [x] switch to the app-content when an entry in app-navigation is clicked (for navigation)
* [x] decide where to put a toggle button (using the menu.svg icon) so it’s more discoverable than just swiping
* [x] test if drag&drop works, maybe adjust so both swiping and drag&drop works
* [x] the hard part: adjust the apps which use app-navigation/app-content but don’t work / modify core apps.css
* [x] test opening files, for example with the text editor
* [ ] check smoothness of animation against demo at http://jakiestfu.github.io/Snap.js/demo/apps/default.html