Adding count of instructors to "all events" page.

There isn't room to display all the instructors next to the event on the "all events" page as requested by #11, so this displays a count instead, and highlights workshops that are missing instructors.

1.  Modify `all_events` function to decorate event objects with number of instructors.
2.  Modify view to display those counts.
3.  Modify base `_page.html` to load CSS to highlight events with no instructors.