Normalize named entities to numeric

Normalize named entities to numeric

We should never be outputting named entities other than the ones in XML,
< > & ", because that will break well-formedness unless
we have a DTD in the doctype, which we don't in HTML5 mode.

I stuck with outputting numeric entities here instead of UTF-8 because
some characters are hard to read in UTF-8 (e.g.,  ).  Maybe it
would be nicer if we decoded to UTF-8 except for whitespace and control
characters, or something like that, but it's a detail.

I'll backport to 1.17 and add RELEASE-NOTES there, which is why I added
the line to HISTORY instead of RELEASE-NOTES.