[NEW] Links in static interactives

[Rob Beezer]

Interactive components (#interactive, #video, #audio) still have experimental
static representations as a #sidebyside with an image, a QR code, and a list of
links.

The list of links now includes an "in context" link to the location in your
project, in addition to the "standalone" page. That is new. There is also an
"Embed" link that will be discussed on pretext-dev just now. Turns out it is
hard to know how HTML is chunked when you are making static output formats.

I've put these objects into #sidebyside, but since the static version is a
#sidebyside, we end up with nested #sidebyside, which is not legal PreTeXt. I
expect to adjust the schema, if it has not been already.

As this settles down, we should add publisher options for which links to show,
and which link goes into the QR code. So expect some more improvements and
iterations, this is not final (still!).

Recent improvements have been motivated by work by Andrew Scholer, who will be
bringing a similar treatment to interactive #program (ActiveCode and CodeLens).

Rob