Add new Mode class and test stubs for it

frontend/src/metabase-lib/lib/Mode.js

frontend/src/metabase-lib/lib/Mode.spec.js

frontend/src/metabase-lib/lib/Question.integ.spec.js

frontend/src/metabase-lib/lib/Question.js

frontend/src/metabase-lib/lib/Question.spec.js

frontend/src/metabase-lib/lib/queries/Query.js

frontend/src/metabase/qb/lib/modes.spec.js

...d/src/metabase/query_builder/components/ActionsWidget.jsx

frontend/src/metabase/query_builder/selectors.js

.../src/metabase/visualizations/components/Visualization.jsx