move Help section from controls bar to sidebar

(The horizontal iFrame scrollbar is the same on current master – this is simply a move from controls to the new app style.)

Possible future enhancements (not blocking for this PR):
* [ ] either iframe the Bugtracker, Forum as well or mark them as external links with an icon
* [ ] mark the active navigation entry in the navigation

Please review @karlitschek @owncloud/designers