Make current_student method more robust

There have been several cases where students drop their databases and
then can't figure out why their app is throwing errors in
current_student.
Let students create an attendance
Let students see all attendances

Since we reduced the scope of the 4th assignment, we're adding an 
intermediate step to show that attendance was successfully created.
Merge branch 'assignment-4'
Let students create an attendance
Add failing feature spec for student seating

The student index page will now show students either sitting in one of
the four quadrants or as absent.
Render students based on their attendance
Use present to define in_seat
Validate only one attendance per student per day
Set attendance attended_on in controller
Add numericality seat validations
Add some styling for present students
Clarify wording in spec
Fix spec failures after crazy rebase
Configure timezone
Merge branch 'assignment-5'
Update student spec to catch more errors
Use subquery for absent

This should keep more of the finding in SQL with a subquery instead of
examining IDs in Ruby.