Don't enqueue experiments.js when no experiments are in use

[johnbillion]

The experiments.js file is always enqueued. It would be nice if this was only enqueued when something with an experiment is loaded on the page, to reduce page weight.

[roborourke]

We would have to enqueue the script just in time but right now we load it super early in the head to avoid things jumping around on the page and so that the custom web components can render immediately.

It's a trade off and the current behaviour is intentional but could possibly be made optional. Needs research to decide if we actually want to do it and if it'll negatively affect the behaviour of the site and the feature.