add contents webservice

a work in progress, but mostly working at this point.

This mainly renames the 'notebooks' service to 'contents' and teaches it about regular files. There is no longer special treatment of directories, just different forms for `content`. Tree view now shows all files.

As usual, modeled on GitHub [contents API](https://developer.github.com/v3/repos/contents/).

I will enumerate the changes to the models and API, but the gist:

- all contents models have name, path, dates, type
- type is one of: directory, file, notebook
- if content is requested:
  - additional `format` key will be `json`, `text`, or `base64`, describing `content`
  - `content` will be
    - list of JSON models for directory contents
    - notebook JSON for notebooks (format=json)
    - file text for text files (format=text)
    - b64-encoded data for binary files (format=base64)