HTML API: Add TEMPLATE and related support in HTML Processor.
As part of work to add more spec support to the HTML API, this patch adds
support for the IN TEMPLATE and IN HEAD insertion modes. These changes are
primarily about adding support for TEMPLATE elements in the HTML Processor,
but include support for other tags commonly found in the document head, such
as LINK, META, SCRIPT, STYLE, and TITLE.
Developed in https://github.com/wordpress/wordpress-develop/pull/7046
Discussed in https://core.trac.wordpress.org/ticket/61576
Props: dmsnell, jonsurrell, westonruter.
See #61576.
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git-svn-id: http://core.svn.wordpress.org/trunk@58229 1a063a9b-81f0-0310-95a4-ce76da25c4cd