Remove the XSLT xml-stylesheet processing instruction from all sitemaps and sitemap indices...

[pbiron]

In addition to removing xml-stylesheet PI to render the sitemap or index into HTML client-side, this PR adds the ability the render the HTML version of the sitemaps and sitemap indices server-side when a user uses the '.html' extension on any sitemap or sitemap index URL.

In it's current form, this PR does not yet address any necessary changes to unit tests.

Trac ticket: https://core.trac.wordpress.org/ticket/65593

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