Automate Warning banner

[niccokunzmann]

The warning banner at the top of the docs can be automated with JavaScript and does not need to be static.

• latest should show
  
• stable should show nothing
• https://icalendar.readthedocs.io/en/<anything else>/ should show
  
• Bonus: When you run make livehtml, show a warning banner that this is a local development version.

Please write a script that does this.
Also make sure this works on all versions.

[stevepiercy]

I really don't want to mess with the Sphinx theme. I've done that before, and I've been bitten when the upstream changes.

Beside the theme already provides this feature. We're just not managing it correctly. We can manage it as I proposed in #1194 (comment).