Improve overridden schedule slot CSS.
Improve overridden schedule slot CSS.

  - Vertically align all schedule cells in the middle so that multi-hour
    events display in the middle of their cells, not at the top.
  - Since overridden slots automatically include paragraph elements,
    reduce slot paragraph line height to 20px to better fit in with
    other schedule items.

pydata/conf_site,pydata/conf_site,pydata/conf_site