Disallow interactive content in <summary>.

This changes the content model for <summary> to have the same restriction on interactive content that is present for the content model of <button>.  This affects conformance for documents but not for user agents.

Fixes whatwg#2272.

source

@@ -59639,7 +59639,9 @@ interface <dfn interface>HTMLDetailsElement</dfn> : <span>HTMLElement</span> {
    <dt><span data-x="concept-element-contexts">Contexts in which this element can be used</span>:</dt>
    <dd>As the <span>first child</span> of a <code>details</code> element.</dd>
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
-   <dd><span>Phrasing content</span>, optionally intermixed with <span>heading content</span>.</dd>
+   <dd><span>Phrasing content</span>, optionally intermixed with <span>heading content</span>,
+   but there must be no <span>interactive content</span> descendant and no descendant with the
+   <code data-x="attr-tabindex">tabindex</code> attribute specified.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
    <dd><span>Global attributes</span></dd>
    <dt><span