Add course-specific URL overrides for social sharing icons on student dashboard

cms/djangoapps/models/settings/course_metadata.py

cms/envs/bok_choy.env.json

cms/envs/common.py

common/lib/xmodule/xmodule/course_module.py

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

common/test/acceptance/pages/studio/settings_advanced.py

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

lms/envs/bok_choy.env.json

lms/envs/common.py

lms/templates/dashboard/_dashboard_course_listing.html