Addressed notes from reviewers on Library Pagination.

cms/djangoapps/contentstore/views/item.py

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

cms/static/js/views/library_container.js

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

cms/static/sass/elements/_pagination.scss

cms/static/sass/elements/_xblocks.scss

.../js/mock/mock-container-paged-after-add-xblock.underscore

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

common/lib/xmodule/xmodule/library_root_xblock.py

common/lib/xmodule/xmodule/video_module/video_handlers.py