Implement a save button for open ended responses

common/lib/xmodule/xmodule/combined_open_ended_module.py

...on/lib/xmodule/xmodule/css/combinedopenended/display.scss

...b/xmodule/xmodule/js/src/combinedopenended/display.coffee

...pen_ended_grading_classes/combined_open_ended_modulev1.py

...e/xmodule/open_ended_grading_classes/open_ended_module.py

...dule/xmodule/open_ended_grading_classes/openendedchild.py

...dule/open_ended_grading_classes/self_assessment_module.py

common/lib/xmodule/xmodule/tests/test_combined_open_ended.py

common/lib/xmodule/xmodule/tests/test_self_assessment.py

lms/templates/combinedopenended/openended/open_ended.html

...binedopenended/selfassessment/self_assessment_prompt.html