Integrate Fetch into HTML

This is not a perfect rewrite, but provides a much more solid basis for implementations and future work. Follow up fixes will be needed to create a more uniform whole. (See #151, #156, and #158.)

Fixes:

* https://www.w3.org/Bugs/Public/show_bug.cgi?id=24080
* https://www.w3.org/Bugs/Public/show_bug.cgi?id=24613
* https://www.w3.org/Bugs/Public/show_bug.cgi?id=24659
* #95
* #106
* #140

Author: annevk