Allow url param override on app.rerun.io

[jleibs]

The logic to provide a default url from javascript was preventing the old ?url param behavior from working. If url param is provided we now pass that through directly.

The other option would be for web.rs to always default to the url param if provided, but I can imagine other cases where the hosting web app wants full control over the url-param, so I think this is the preferable way of solving the problem.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)