Move overview drag and drop code to a utility class.

cms/static/coffee/spec/main.coffee

cms/static/coffee/spec/views/overview_spec.coffee

cms/static/js/spec/utils/drag_and_drop_spec.js

cms/static/js/utils/drag_and_drop.js

cms/static/js/views/overview.js

cms/templates/js/mock/mock-outline.underscore