EDUCATOR-526 Update view cert button to only appear after availability

common/djangoapps/student/tests/tests.py

common/lib/xmodule/xmodule/course_metadata_utils.py

common/lib/xmodule/xmodule/course_module.py

...n/lib/xmodule/xmodule/tests/test_course_metadata_utils.py

lms/djangoapps/certificates/tests/test_api.py

lms/djangoapps/courseware/tests/test_views.py

lms/djangoapps/courseware/views/views.py

lms/djangoapps/mobile_api/testutils.py

lms/templates/courseware/progress.html

...rations/0014_courseoverview_certificate_available_date.py

openedx/core/djangoapps/content/course_overviews/models.py