Backport PR
#5584: more semantic icons
This adds classes to customize the folder and notebook document icons in the dashboard view.
I've also changed the notebook icon to have it better distinguished from the
folders and match e.g. Github's approach to this issue.
For the pixel-perfectionist, I've also made these icons a fixed width (they weren't before, I'm sure, like me, it's made you angry ; )
screenshot is coming.
closes
#5555
