Added students.CourseAssignment get_absolute_url() tests and implementation.
Added URL for statistics.show_assigments_stats
Added tests for show_assignments_stats() and implementation
Added statistics.show_assignment_stats() template
Optimized queries in statistics.show_assignments_stats
Removed import_users_from_csv tests for now.
They're written to fail.
Merge branch 'users_stats' of github.com:HackBulgaria/Odin

Conflicts:
	students/tests_commands.py