Simplify schedule cell display logic.
Simplify schedule cell display logic.

If slot has an attached presentation (slot.content will return True),
display presentation and speaker information. Otherwise, if slot has a
content override, display that. Otherwise, display the label of the slot
kind.

This eliminates the need to have hardcode slot kind labels (see #69) and
ensure that content overrides will display properly.

pydata/conf_site,pydata/conf_site,pydata/conf_site