revised cheatsheet markdown, fixed height + added vertical scroll, and replaced explanation example in sheet

on server

This cuts the number of queries in 6.002 courseware loads from ~650 to
~5-40. Still to do: cache CustomTag templates so that we only load them
once per request.

in rounds down times from :01-:09 to :00 (and perhaps others). I think
that's a bug or setting in timepicker.