quote data attributes

prevents errors when these are empty strings
add utils.js to tree template
add utils.url_path_join

javascript equivalent of Python-side function.
logo link to current tree level

rather than top level
update json_errors decorator

separate handling of HTTPError and unhanded exceptions
fix `--notebook-dir` configurable when there is no trailing slash
adjust definition of 'path' in notebooks

never includes leading or trailing '/'
move url_[un]escape to utils from nbm
mixup notebook_list
move os_path to FileNBMan

doesn't belong in base NotebookManager
log exceptions parsing JSON
fix dashboard upload
cleanup old, unused `/notebooks/foo` API methods

these have moved to `/api/notebooks`