Show checkmarks when "Show Answer" is clicked.

common/lib/xmodule/xmodule/capa_base.py

common/lib/xmodule/xmodule/js/spec/capa/display_spec.coffee

common/lib/xmodule/xmodule/js/src/capa/display.js

common/test/acceptance/pages/lms/problem.py

common/test/acceptance/tests/lms/test_problem_types.py