Add mock graphic separators

To illustrate where we're putting graphic elements, add an
example of a courtroom sketch and gray separator bars.

I think the graphic elements look better if they take up the full
viewport width, so remove the padding from the body and instead
use the class column in the Tarbell spreadsheet to add the
Bootstrap ``container`` class.

Break the graphic elements into their own "slides" so they can
be contstrained within a container or take up the full width at
our discretion.