Improve scores views

Added two new views and extracted scores using single queries.
Files changed:
config/routes.rb ->added routes for the below added views
app/models/assignment.rb ->refactored get_scores method
app/controllers/grades_controller.rb ->added two new methods, all_scores and view_course_scores
Files added:
app/views/all_scores.html.erb ->displays scores of all assignments for a user
app/views/view_course_scores ->displays scores for all assignment and users for a course