Hierarchical notebook names

This pull request implements hierarchical folder names in the notebook list. If you say `some folder name/notebook 1` then RCloud will do what you expect, which is to create a folder for that notebook. If there 

@gordonwoodhull Do you mind taking a look at this before I merge it? I think it's clean and does the right thing. The one annoying bug that exists now is that if you move files into or out of folders, the node does not get moved to the right place. You need to reload the RCloud page to get it to work. This is because `update_tree_entry` only changes one tree at a time. I don't want to muck around with jqtree too much, so I think that an acceptable workaround would be to detect when there are changes to the tree structure (moving things across folders), and in that case we nuke the whole tree and recreate it. Do you think that's feasible?