Remove %webroot% from CSS

Move from the performance-killing %webroot% in CSS to instead have an icons.css file where apps can refer icons from core directly via HTML classes.

This pull request includes the icons file and cleaning up of the apps in the core repo. Still to follow is cleaning up of all apps and actually ripping out the webroot and appswebroot substitution method from core.

Please review @DeepDiver1975 @PVince81 @owncloud/designers 

Btw @butonic I removed the .toggle rule in upload.css because it seemed to get used nowhere. Let me know if that’s wrong: https://github.com/owncloud/core/compare/replace-webroot-in-css?expand=1#diff-1e59de7cf00a99e45afecad263368216L122