Adding day_activities function to classrooms.

This returns all of the activities that will happen in the class on a
given day.
Add classes_with_assignments_on function to user

This will return all of the user's classes which have activities on a
given day.

I realized that I should rename it to classes_with_activities_on,
considering that that is what it selects based on. I'll do that right now.
classes_with_{assignments -> activities}_on in user

(Renaming the function classes_with_assignments_on to
classes_with_activities_on to better reflect the behavior of the function)
Some changes to how docket is generated.

Each assignment or exam is linked to within the document, and only
classes with things on a given day will be displayed.