Support xblocks with children on the container page.

cms/djangoapps/contentstore/views/component.py

cms/djangoapps/contentstore/views/item.py

cms/djangoapps/contentstore/views/preview.py

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

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

cms/static/sass/elements/_xblocks.scss

cms/templates/container.html

cms/templates/container_xblock_component.html