Lazy-load InstantSearch scripts and stylesheets

[iBug]

trafficstars

This is an enhancement or feature.

Summary

So that Algolia's InstantSearch-related stuff is only loaded from CDN when user clicks on the search icon. Those files add up to 400 KiB and are currently loaded in a blocking way, which has a non-trivial performance impact.

You can inspect the effect on my website.