Refactor Django App Plugins to allow for additional features

cms/envs/aws.py

cms/envs/common.py

cms/envs/devstack.py

cms/envs/test.py

cms/urls.py

lms/djangoapps/grades/apps.py

lms/envs/aws.py

lms/envs/common.py

lms/envs/devstack.py

lms/envs/test.py

lms/urls.py

openedx/core/djangoapps/ace_common/apps.py

openedx/core/djangoapps/plugins/README.rst

openedx/core/djangoapps/plugins/__init__.py

openedx/core/djangoapps/plugins/constants.py

openedx/core/djangoapps/plugins/plugin_apps.py

openedx/core/djangoapps/plugins/plugin_settings.py

openedx/core/djangoapps/plugins/plugin_urls.py

openedx/core/djangoapps/plugins/registry.py

openedx/core/djangoapps/plugins/utils.py