Fix panel preview rendering on JupyterHub 4.1+

[krassowski]

• Fixes https://github.com/holoviz/panel/issues/7039
• Sibling to https://github.com/bokeh/bokeh/pull/14003

Briefly, when running on JupyterHub this PR:

• loads the panel preview into srcdoc instead by using src as srcdoc is not subject to CSP
• exposes information about the URL so that Bokeh can adjust the websocket URL accordingly

Before	After

How to test without installing JupyterHub

1. Install bokeh with https://github.com/bokeh/bokeh/pull/14003 applied 2. Run `jupyter --paths` and choose a writable config path from the `config:` section, e.g. `/home/username/my-env/etc/jupyter` 3. Open (create if needed) a `/home/username/my-env/etc/jupyter/jupyter_server_config.py`, paste the following: ```py c.LabApp.tornado_settings = { 'hub_prefix': '/hub', 'hub_host': 'test', 'user': 'test', 'headers': {'Content-Security-Policy': "frame-ancestors 'none'"} } ``` 4. Start `jupyter lab` 5. Open dev tools Network tab, click on a request and expand Response Headers, verify that CSP for frame ancestors is set to none:  6. Ensure that page data includes `hubPrefix` by viewing the source of JupyterLab page  7. Follow reproduction steps in https://github.com/holoviz/panel/issues/7039

[krassowski]

The build failure is unrelated - I opened https://github.com/holoviz/pyviz_comms/issues/126 and two PRs with proposed solutions.

[philippjfr]

Really appreciated! Will test and then cut a release asap.