Mobile optimization for base layout and Files app

Stuff left to do:

* [ ] hide Rename and Versions. For that the file actions need classes like ```action-rename``` etc. @DeepDiver1975 need your help. :)
* [ ] make the menu toggle for the navigation only work on mobile – it shouldn’t be possible to toggle the navigation on desktop. @PVince81 can you help here?
* [ ] ideally use [Headroom.js](http://wicky.nillia.ms/headroom.js/) so the header and controls bar is hidden on scrolling down, but immediately shown on a bit of scrolling up. Important to preserve space on mobile

And probably some more stuff I missed.

@karlitschek please check if the approach is fine in general.