This patch extracts the 'nonce' attribute out to a generic definition in
the "Fetching resources" section (alongside "CORS settings attributes",
etc.), and defines some new behaviors with the intent of reducing the
risk of side-channel leakage of the nonce's value.

In short, the nonce value is extracted from the content attribute when
the element is inserted into the DOM, and put into an internal slot. The
content attribute's value is set to the empty string.

From then on, the slot's value and the content attribute's value are
disconnected; alterations to one have no effect on the other, and
vice-versa.

The nonce's value is available to script via the `nonce` IDL attribute,
and so can be propagated just as today.

Addresses #2369.