Refactor students#show.
Add 'students' to CourseData.
Use `current_course_data` in views.
Fix error in Students#Index.

Closes #405.
Move queries to `current_course_data.`
Parameterize query options.
Continue using `current_course_data` in views.
Clean up students#show and fix `sorted_students`.
Fix typo in students#leaderboard.