Further refactoring and refinements fo html templates

core/templates/core/r/feedback_list.html

core/templates/core/r/reviewer_base.html

core/templates/core/r/reviewer_startpage.html

core/templates/core/r/single_submission.html

core/templates/core/r/student_submission_list.html

core/templates/core/s/single_submission.html

core/templates/core/s/student_startpage.html

core/urls.py

core/views/feedback.py

core/views/submission.py

core/views/user_startpages.py