[CHANGE] Interactives and side-by-sides

[Rob Beezer]

#audio and #video are lumped in with #interactive, as things that do not do very
well in static (non-HTML) formats. In those cases these all get converted to a
manufactured #sidebyside with a screenshot, QR code, and links. That
*manufactured* #sidebyside will not do very well if it sits inside an *authored*
#sidebyside.

So the schema and validation-plus stylesheet have been adjusted to preclude
#audio, #video, #interactive from being a *descendant* of a #sidebyside.

So, for example, a #video inside a #figure is a very good (and common)
construction, but it cannot become the panel of a #sidebyside. No actual code
has changed (yet!), but you will now start to see deprecation warnings.

One edge case of note: an #exercise can go in a #sidebyside, but only if it is
within a #worksheet. An #exercise has been allowed within a #stack panel in the
same situation, but had never really been tested. With these changes, this
construction is not supported. We could revisit that decision if there is demand.