Various improvements to the class changes grid

* Allow displaying all data all the time, instead of just in popups
  (Partial fix to #56 and #62; inspired by what appears without the
  previous commit applied)
* Put enrolled classes at the top (#58)
* Show prereqs, difficulty levels in tooltips (#59)
* Minor code cleanup
Allow showing/hiding walk-in seminars (#289)

This starts with them hidden. It's not clear that that's really ideal.
Correct year question
Explanatory note about year numbering
Spelling fix
Merge pull request #290 from dehnert/onsite-fixes

Allow showing/hiding walk-in seminars (#289)