Require UTF-8

This change adds a “must” requirement for UTF-8 in all but one of the places in
the spec that define a means for specifying a character encoding.

Specifically, it makes UTF-8 required for any “character encoding declaration”,
which includes the HTTP Content-Type header sent with any document, the `<meta
charset>` element, and the `<meta http-equiv=content-type>` element.

Along with those, this change also makes UTF-8 required for `<script charset>`
but also moves `<script charset>` to being obsolete-but-conforming (because now
that both documents and scripts are required to use UTF-8, it’s redundant to
specify `charset` on the `script` element, since it inherits from the document).

To make the normative source of those requirements clear, this change also adds
a specific citation to the relevant requirement from the Encoding standard, and
updates the in-spec IANA registration for text/html media type to indicate that
UTF-8 is required. Finally, it changes an existing requirement for authoring
tools to use UTF-8 from a “should” to a “must”.

The one place where this change doesn’t yet add a requirement for UTF-8 is for
the `form` element’s `accept-charset` attribute. For that, see issue #3097.