Added ability to disable xblock types in LMS.

cms/envs/common.py

common/djangoapps/xblock_django/admin.py

common/djangoapps/xblock_django/migrations/0001_initial.py

common/djangoapps/xblock_django/migrations/__init__.py

common/djangoapps/xblock_django/models.py

common/lib/xmodule/xmodule/hidden_module.py

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

common/lib/xmodule/xmodule/modulestore/django.py

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

common/lib/xmodule/xmodule/modulestore/split_mongo/split.py

common/lib/xmodule/xmodule/x_module.py

lms/djangoapps/courseware/tests/test_module_render.py

lms/envs/common.py

lms/static/sass/course/courseware/_courseware.scss

openedx/core/lib/xblock_utils.py