* convert UIS to a jupter server extension application

* put extension behind cylc app
* deal with namespace package issues
* allow args to be passed through to jupyter_server

* this is for the workflow owner not the authenticated user

* logging -> self.log

* tighter integration with jupyter_server
* less bespoke static logic

* setup.py -> setup.cfg
* pytest.ini -> pyproject.toml
* pypi classifiers

* revert to decorator approach

* standardise flake8 setup with cylc flow
* fix violations

* Closes cylc#231
* Provide our own static handler rather than using Jupyter Server because:
  * Our static files are contained in sub dirs, Jupyter Server does not
    support this for static files.
  * We are not using the same template/static resource approach.
  * We would have to edit URLs in the UI to point at `static/cylc/*`.
  * We are opening these static files to multiple users so need to push
    them through our authorisation code.
* Adding authentication here provides login redirection and does all the
  cookie / token stuff.

* Add an endpoint that provides the equivalent to `cylc version --long`
* This replaces the `hello` handler used during development.
* As it doesn't engage any extra logic it may be useful for debugging
  UI/UIS issues.
* It may also be useful for debugging environment issues.
* Document handlers.