Fix bug in searching for students viewed in last N days

Also refactored a bit of code:
-- Added a new findByViewedLastNDays method
-- Added/improved some tests
-- Rewrote some FunSpec tests as FreeSpec tests
-- Removed the warning on calculated units not matching database units
(which was bogus, I believe)
Merge branch 'MinusNBugfix' into 2.1