[web] Update jquery-treeview to the latest version
Files copied from commit 61f230828ace1b54c2cc54bc549ece261b11842e of
https://github.com/jzaefferer/jquery-treeview
[web/css] move jquery.treeview.css override to a separate file
Patched embedded code copies are a maintenance disaster (even more so
than plain embedded code copies). Let's not do that.