Updated empty state for segments, adjusted header borders

frontend/src/metabase/icon_paths.js

frontend/src/metabase/reference/Reference.css

...end/src/metabase/reference/containers/ReferenceEntity.jsx

.../metabase/reference/containers/ReferenceRevisionsList.jsx

frontend/src/metabase/reference/selectors.js

resources/frontend_client/app/img/lightbulb.png

resources/frontend_client/app/img/lightbulb@2x.png

resources/frontend_client/app/img/segments-list.png

resources/frontend_client/app/img/segments-list@2x.png