MultiQuery class and related refactorings

frontend/interfaces/underscore.js

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

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

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

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

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

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

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

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

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

...tend/src/metabase/containers/AddToDashSelectDashModal.jsx

frontend/src/metabase/meta/Card.js

frontend/src/metabase/meta/types/Card.js

frontend/src/metabase/qb/components/modes/TimeseriesMode.jsx

frontend/src/metabase/query_builder/actions.js

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

...rc/metabase/query_builder/components/CardFiltersWidget.js

...tend/src/metabase/query_builder/components/MetricList.jsx

.../src/metabase/query_builder/components/QuestionEditor.jsx

...metabase/query_builder/components/SavedMetricSelector.jsx