[WIP] Mobile sidebar swipe

:construction:

This is the work-in-progress branch to make ownCloud more usable on mobile. Specifically this is about having a way to toggle between the sidebar (app-navigation) and content (app-content). This currently works via swipe.
**For now this only works with the Apps management, since other apps unfortunately modify core CSS** – which needs to be changed.

TODO:
* switch to the app-content when an entry in app-navigation is clicked (for navigation)
* decide where to put a toggle button (using the menu.svg icon) so it’s more discoverable than just swiping
* the hard part: adjust the apps which use app-navigation/app-content but don’t work / modify core apps.css

It’s a bit unfortunate to see that so many of the apps which use custom modifications on top of the core apps.css – that makes it hard, or impossible to have this all compatible on mobile.
@Raydiation @cosenal @tanghus @jbtbnl would be cool if you can look into that, and we can settle on one definitive way to do the sidebar. cc @owncloud/designers