For Datasette Cloud I want the ability to have one Datasette instance serve different templates depending on the host of the incoming request - so I can have a private simon.datasette.cloud instance running default Datasette, but I can let users have a public simon.datasette.site instance which uses their own custom templates.

I don't want people running custom templates on *.datasette.cloud for security reasons - I dont want an XSS hole in a custom template being able to steal cookies or perform actions on behalf of signed-in users.

I tried implementing this at first with a monkeypatch, but ran into problems. I'm going to instead do a research spike to see if a plugin hook that allows plugins to influence the Jinja environment based on the incoming request is a clean and better mechanism for this.