Atom feed alternate type of html_type, not hardcoded text/html

[niallkennedy]

When a document of one media type links to another it's possible to provide a media type hint through the type attribute.

In the main Atom feed PHP we point to permalinks and the site URL with a hardcoded type=text/html; the Atom comments feed template uses the more correct html_type reference for better accuracy in the media type hint if publishers have varied from the default of text/html.

Patch Atom feed template to use html_type instead of assuming text/html.

[niallkennedy]

html_type instead of text/html

[nacin]

In [21289]:

Use html_type for alternate link elements in atom feeds, as done in atom comment feeds. props niallkennedy. fixes #21305.