Convert Course, Section and Sequence XModules to XBlocks. (#25965)

...oapps/contentstore/management/commands/clean_cert_name.py

cms/djangoapps/contentstore/tests/test_contentstore.py

cms/djangoapps/contentstore/tests/test_crud.py

cms/djangoapps/contentstore/utils.py

cms/djangoapps/contentstore/views/course.py

cms/djangoapps/contentstore/views/item.py

cms/djangoapps/contentstore/views/tests/test_course_index.py

common/djangoapps/course_modes/models.py

common/djangoapps/static_replace/__init__.py

common/djangoapps/student/tests/tests.py

common/djangoapps/util/milestones_helpers.py

common/lib/xmodule/setup.py

common/lib/xmodule/xmodule/course_metadata_utils.py

common/lib/xmodule/xmodule/course_module.py

common/lib/xmodule/xmodule/library_content_module.py

common/lib/xmodule/xmodule/library_root_xblock.py

common/lib/xmodule/xmodule/modulestore/__init__.py

common/lib/xmodule/xmodule/modulestore/inheritance.py

common/lib/xmodule/xmodule/modulestore/mixed.py

common/lib/xmodule/xmodule/modulestore/mongo/base.py