rename notebooks service to contents service

minimal functional changes, committing because tests are passing.
teach contents service about non-notebook files
teach tree view about non-notebook files
add support and tests for uploading and saving regular files
test creating a directory with PUT
contents service review

- missed some s/notebook/file/ in docstrings
- manager doesn’t sort contents
support deleting empty directories

can’t copy directories
use contents manager to handle redirect of /notebooks -> /files

avoids 404 on /files for visiting a nonexistent notebook
updates per review

a few more notebook mentions in docstrings,
and inappropriate use of setdefault.