Support arbitrary comparisons, add support for raw data cards too

frontend/src/metabase/components/EntityMenu.jsx

frontend/src/metabase/routes.jsx

frontend/src/metabase/services.js

frontend/src/metabase/xray/components/ComparisonDropdown.jsx

frontend/src/metabase/xray/containers/SegmentComparison.jsx

...d/src/metabase/xray/containers/SegmentTableComparison.jsx

frontend/src/metabase/xray/containers/SegmentXRay.jsx

frontend/src/metabase/xray/containers/TableComparison.jsx

...tend/src/metabase/xray/containers/TableLikeComparison.jsx

frontend/src/metabase/xray/containers/TableXRay.jsx

frontend/src/metabase/xray/xray.js

frontend/test/xray/xray.integ.spec.js