Add overall SRI (Subresource Integrity) support to verify CDNs delivering the correct files

[swernerx]

This is mainly a security feature which verifies whether files delivered through any 3rd party server math the checksums detected during deployment.

This requires adjustments to the chunk flushing in our React server as well a possible some tweaks for dynamic chunk loading in Webpack e.g. when using import().

See also:
faceyspacey/webpack-flush-chunks#10 (comment)