build-slide chokes on 4th-level headings

[schanzer]

We almost never use them outside of some recent changes to Algebra 2, so it's no surprise this hasn't come up before.

In exploring-linear-models, the content around 4th level headers just doesn't show up in zlides.md.

• These headers should be ignored by slides altogether
• Once done, ES needs to bring them back to the Alg2 lessons.

[flannery-denny]

@ds26gte

• In @schanzer's spec it says "4th-level headers should be ignored by slides all together"
• Your commit it titled "==== sections should become @slidebreaks"

Having read @schanzer's spec I inserted slidebreaks where I needed them in the box-plots lesson. When I built slides I got synthesize templates in the launch section.

I have resolved the issue in the deck I was working on by proscribing the template with each slidebreak, but this is a tricky thing for authors to catch. please update lua so that ==== are ignored by slides so that authors can decide where to break slides and we end up with slides templates that correspond to the lesson section we're authoring in.

[flannery-denny]

@ds26gte Noting that even with @slidebreak{LaunchC} on line 279 of the lesson plan I had to manually go select the correct template in google drive for this one.