Added "collapse" class to sidebar folders.

It seemed to work fine without that class, but according to the bootstrap docs it should be added to the collapsible element.
Changes in the sidebar to make it work like an accordion.

A bit of mixing between panel-group and list-group markups is used. It seems a bit weird but it seems to work fine. I was happy using list-groups but I haven't been able to make the accordion example in the bootstrap docs work with list-groups, only with a panel-group.