Fix resource loading edge cases

[kkaefer]

This PR bundles a few fixes for edge cases in resource loading:

• Changes URLs in our debug pages to be relative, so that we can test them when loaded via the file:// URL scheme as well (as long as they don't depend on other local resources)
• Fixes a bug that broke map loading when the document containing the map view was loaded from a blob URL: Referrer error in iframe #8069
• Classifies relative URLs as file:// URLs when the original document was loaded from a file URL to avoid using Fetch for those.

[kkaefer]

Things to test:

• Are we sending correct referrers in all browsers?
• Does the new debug/iframe-blob.html page work in all browsers?
• Does the debug/index.html page work when loaded via a file:// URL?

[asheemmamoowala]

Does the new debug/iframe-blob.html page work in all browsers?

Verified on OSX Safari/Chrome/Firefox, Android Chrome/Firefox, and Windows Edge.
Did not seem to work on IE11 😢

Does the debug/index.html page work when loaded via a file:// URL?

✅

[kkaefer]

IE doesn't support HTML documents loaded from data or blob URLs.

[kkaefer]

Changelog entry:

• Fix map loading for documents loaded from Blob URLs
• Improve classification of relative URLs as file:// URLs when the document was loaded from a file URL