lab: open log files in Jupyter Lab

[oliver-sanders]

Training day exercise: Write a Jupyter Lab plugin

• Closes JupyterServer: opening content in other extensions cylc-uiserver#472
• Offers an "open in Jupyter Lab" button in the Log view (when Jupyter Lab is installed).
• Works with https://github.com/oliver-sanders/cylc-jupyterlab-extension

How it works:

• The user is presented an "Open in Jupyter Lab" button in cylc-ui.
• When pressed, cylc-ui sends a REST request to the URL lab-bridge/open/<file>
• When this request is received, cylc-uiserver logs an "event" with Jupyter Events.
• This event is bubbled up to all components (incl, Lab).
• A Jupyter Lab extension listens for this event and opens files when it receives one.

Components:

• cylc-ui
• cylc-uiserver
• jupyterlab extension

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.