Try to test Cylc UI Server with experimental Tornado with HTTP/2

[kinow]

Describe exactly what you would like to see in an upcoming release

There's a repository created by one of the maintainers of Tornado, that adds support to HTTP/2. But it hasn't received much feedback yet, which is a blocker to this feature being incorporated someday into Tornado.

It would be nice to get it so that Cylc UI multiple resources are served in batch via HTTP/2 instead of individually via HTTP/1.1.

Additional context

See the lighthouse/chrome audit performance output for the note about HTTP/2. Plus it's always nice to give back to the OSS we use if we have some spare time.

https://github.com/tornadoweb/tornado/issues/1438

Pull requests welcome!