add jupyter extensions to userprofile handler

[oliver-sanders]

Partially addresses #470

List Jupyter server extensions that provide web applications (i.e. cylc & lab) in the "userprofile".

• jupyter lab dashboard link cylc-ui#1982 uses this to add a Jupyter Lab button to the dashboard page of the Cylc UI.
• A Jupyter Lab plugin might use this to add a button to Cylc (and any other Jupyter web apps) into the Jupyter Lab menu, similar to Running JupyterLab alongside the Cylc UI #298 (comment).

I might see if I can get the implementation into Jupyter Server in the future (opened jupyter-server/jupyter_server#1469). We will probably want to continue merging this into our own userprofile endpoint for convenience though.

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 (it doesn't)
• 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.