adjust to new core app-styles

This adjusts the HTML (structure, classes, IDs), CSS and JS to the new app styles coming to core. For this to work, the pull request https://github.com/owncloud/core/pull/2275 needs to be reviewed and pulled in first! Also, pulling this in will break compatibility with the 5.0 series, unless the above mentioned pull request will be backported.

Please review @icewind1991 @caco3 @Raydiation