Add animations for born hidden and xref knowls

[ascholerChemeketa]

This should be ready to roll.

Adds animations back that were lost in the transition to details/summary and then in the conversion of knowls.js away from jquery.

@davidfarmer @Alex-Jordan

[rbeezer]

Took this for a quick drive around the block.

"xref" knowls, and hint|answer|solution always-born-hidden knowls are working great (to my eye).

But a larger block, electively knowled, opens without any animation and then cannot be closed. In the sample article Checkpoints 4.4, 4.7, 4.14. Firefox and Chrome (latter might not be super-recent). A hint|answer|solution inside these Checkpoints still seems fine.

In the console I see:

Uncaught TypeError: this.contentElement is null
    onClick file:///tmp/s/_static/pretext/js/lib/knowl.js:58
    SlideRevealer file:///tmp/s/_static/pretext/js/lib/knowl.js:44
    SlideRevealer file:///tmp/s/_static/pretext/js/lib/knowl.js:44
    addKnowls file:///tmp/s/_static/pretext/js/lib/knowl.js:18
    <anonymous> file:///tmp/s/_static/pretext/js/lib/knowl.js:5
    EventListener.handleEvent* file:///tmp/s/_static/pretext/js/lib/knowl.js:4
knowl.js:58:7

[ascholerChemeketa]

Fixed. Code assumed that all the BHKs were placing contents in div's, but the exercise element has its content in an article.

Now the assumption is that the content is whatever one element appears right after the <summary> inside the <details>. I think that handles all existing BHKs. Requiring there to be one element that contains all of the content for the knowl seems reasonable - pretty much necessary for styling as well.

[rbeezer]

Very good! Combined commits, merged and pushed.

Your call: heads-up to -dev to warn about the change?

There is a "body" template that employs a "body-element" template for making content of many-many things (eg sans title). I think that abstraction gurantees the single-element assumption. But to be safe I am going to comment the code a few places to that effect.

[rbeezer]

Postscript. Should an image description animate? Not a knowl, but is a details. Example:

Figure 9.17 https://pretextbook.org/examples/sample-article/html/section-graphics.html#figure-sage-exosagevec1

[ascholerChemeketa]

@rbeezer Maybe? Seems reasonable.

How about footnotes? Are there other uses of details/summary?