Pull schedule table header into a partial.

Clean up the rendering slightly.  Added `hours_for_day` helper to replace
the list of hours in the table header.
Add unit test for ScheduleHelper#hours_for_day.
Tutor schedule for day.

Tutor::TutorDaySchedule is intended to simplify the view logic listing a
tutor's full day schedule.  It is an iterator that yields hashes, each
corresponding to a single cell in the schedule table.  These hashes detail
the start time, length in cells, and type of block, so that the view code
doesn't have to care.