Prevent the use of summary elements that create accessibility failures

According to a technical reference, any phrasing content is valid within a summary. Notably, this does not include links, though it does include elements like that are also classified as interactive and I would assume therefore should also not be valid.

An argument could be made that this module doesn't itself create links within the summary, it just doesn't prevent an author from adding links, buttons, or other interactive elements there. I believe it should be possible to restrict interactive elements from being put into the summary (via the WYSIWYG, at least). I'm tempted to make this a configurable feature, with accessibility compliance enforced as a default, but I may spin the configurability off into a separate issue.

For completeness:

@mandclu reading on in the mentioned "phrasing content" list you'll find elements with specific conditions.

<a>, if the href attribute is present