Specifically, this makes `<link>` elements with an `integrity` attribute
actually work. Previously, we would load their resource, and then drop
it on the floor without actually using it.
The Subresource Integrity code is in `LibWeb/SRI`, since SRI is the name
of the recommendation spec: https://www.w3.org/TR/SRI/
However, the Fetch spec links to the editor's draft, which varies
significantly from the recommendation, and so that is what the code is
based on and what the spec comments link to:
https://w3c.github.io/webappsec-subresource-integrity/Fixes#18408
