Library setting for show/hide children previews + client-side code to toggle it.

cms/djangoapps/contentstore/views/component.py

cms/djangoapps/contentstore/views/item.py

cms/djangoapps/contentstore/views/library.py

cms/djangoapps/contentstore/views/preview.py

cms/static/js/spec/views/paged_container_spec.js

cms/static/js/spec/views/pages/container_spec.js

cms/static/js/views/paged_container.js

cms/static/js/views/pages/container.js

cms/static/js/views/pages/paged_container.js

cms/static/sass/elements/_xblocks.scss

cms/templates/js/mock/mock-container-page.underscore

cms/templates/js/mock/mock-paged-container-xblock.underscore

cms/templates/library.html

cms/templates/studio_xblock_wrapper.html

common/lib/xmodule/xmodule/library_root_xblock.py

common/test/acceptance/pages/studio/container.py

common/test/acceptance/pages/studio/library.py

common/test/acceptance/tests/studio/test_studio_library.py

lms/templates/studio_render_paged_children_view.html