more semantic icons
remove auth and tree from ipython.less

@minrk said it's the right thing to do
use fixed-width icons, closes #5555
book icon for notebooks, color folders differently

This allows us to use the generic file icon for text files, etc, down
the line.
use folder-close-alt without highlighting

discussion of this happened in #5584
Merge pull request #5584 from ivanov/more-semantic-icons

more semantic icons