Restructure templates

Restructure templates to match new content outline as described in
https://docs.google.com/document/d/1_h6NoyVW_Y1U6mzrtMXm2UiBhIOGjCF4B3_VI2hrGGw

I made the following edits to the template files.  Note that when I say
section, I mean "slide" and when I say "supersection", I mean the three
top-level sections defined in the "sections" worksheet of the Tarbell
spreadsheet.

* Renamed "Explore ..." (explore) supersection to "Behind the scenes"
* (behind-scenes)
* Deleted How a case becomes a case section
* Renamed meet-the-data-sep section to complicated-system-sep
* Deleted "Meet the data" section
* Added "Cook County's top ten" section
* Renamed  "Among drug crimes, ..." (offense-disparities), to "Drug
* crimes make up the bulk of convictions" (drug-crimes)
* Renamed title of "A slice of reality: convictions for sexual and
* domestic violence" to "Convictions for domestic violence and sexual
* assault"
* Added section "DUI convictions are spread across the city" (dui)
* Renamed slide id explore-sep to behind-scenes-sep
* Deleted slide "Windy City convictions by the numbers" (by-the-numbers)
* Added "About the data" (about-data)
* Renamed "Conclusion" (conclude) to "Explore the data" (explore)

When I renamed slides I didn't remove old content.  Feel free to replace
with new content.

When editing existing slides, look for sections like <span
data-term="drugclass">class</span>.  These are the popup definitions and
replicate in new content where appropriate.

When editing existing slides/slides that were renamed, look for <div
class="chart" ...> tags.  These are the placeholders for the charts.
Try to move them to their appropriate place in the new content.  If you
accidently delete it's ok.  I've made notes where they go.

Addresses https://github.com/sc3/cook-convictions/issues/136