[explore] Split large reducer logic in ExploreViewContainer (#3088)

* split reducer logic for ExploreViewContainer

* fix saveModal component and unit tests

* revert changes in SaveModal_spec.
will make another commit just to improve test coverage for SaveModal component.

* remove comment-out code

* fix merge confilicts

superset/assets/javascripts/explore/reducers/saveModalReducer.js

@@ -0,0 +1,28 @@
+/* eslint camelcase: 0 */
+import * as actions from '../actions/saveModalActions';
+
+export default function saveModalReducer(state = {}, action) {
+  const actionHandlers = {
+    [actions.FETCH_DASHBOARDS_SUCCEEDED]() {
+      return Object.assign({}, state, { dashboards: action.choices });
+    },
+    [actions.FETCH_DASHBOARDS_FAILED]() {
+      return Object.assign({}, state,
+        { saveModalAlert: `fetching dashboards failed for ${action.userId}` });
+    },
+    [actions.SAVE_SLICE_FAILED]() {
+      return Object.assign({}, state, { saveModalAlert: 'Failed to save slice' });
+    },
+    [actions.SAVE_SLICE_SUCCESS](data) {
+      return Object.assign({}, state, { data });
+    },
+    [actions.REMOVE_SAVE_MODAL_ALERT]() {
+      return Object.assign({}, state, { saveModalAlert: null });
+    },
+  };
+
+  if (action.type in actionHandlers) {
+    return actionHandlers[action.type]();
+  }
+  return state;
+}