Rebase NEL on top of Fetch

[clelland]

The spec was originally written with abstract "Network requests" which were not well-defined, and references the HTTP RFC for its definition of "request". There are a couple of notes in the spec that suggest things would be simpler if this were to use Fetch concepts directly, as well as some notes in issues:

(See #101 (comment) for instance)

This may exclude some non-browser use cases, but this spec should probably be focused on clients which follow fetch.