Fix panel preview rendering on JupyterHub 4.1+

[krassowski]

• Fixes Panel preview does not work on JupyterHub 4.1+ hosted on a single domain panel#7039
• Sibling to Fix rendering on JupyterHub 4.1: respect absolute_url for protocol determination, support data-absolute-url bokeh/bokeh#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 Fix rendering on JupyterHub 4.1: respect absolute_url for protocol determination, support data-absolute-url bokeh/bokeh#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:

    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 Panel preview does not work on JupyterHub 4.1+ hosted on a single domain panel#7039

[philippjfr]

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