Post follow and updates notifications V1 (#111)

* It is now possible to follow a post in order to receive updates about it
* Notifications are now sent when updates are published
* Post status changes are now tracked
* Update sidebar now shows the post status history
* Mark a comment as a post update using the comment form
* ... more ...

app/javascript/reducers/postStatusChangesReducer.ts

@@ -0,0 +1,71 @@
+import {
+  PostStatusChangesRequestActionTypes,
+  POST_STATUS_CHANGES_REQUEST_START,
+  POST_STATUS_CHANGES_REQUEST_SUCCESS,
+  POST_STATUS_CHANGES_REQUEST_FAILURE,
+} from "../actions/PostStatusChange/requestPostStatusChanges";
+
+import {
+  PostStatusChangeSubmitted,
+  POST_STATUS_CHANGE_SUBMITTED
+} from '../actions/PostStatusChange/submittedPostStatusChange';
+
+import IPostStatusChange from "../interfaces/IPostStatusChange";
+
+export interface PostStatusChangesState {
+  items: Array<IPostStatusChange>;
+  areLoading: boolean;
+  error: string;
+}
+
+const initialState: PostStatusChangesState = {
+  items: [],
+  areLoading: false,
+  error: '',
+};
+
+const postStatusChangesReducer = (
+  state = initialState,
+  action:
+    PostStatusChangesRequestActionTypes |
+    PostStatusChangeSubmitted
+) => {
+  switch (action.type) {
+    case POST_STATUS_CHANGES_REQUEST_START:
+      return {
+        ...state,
+        areLoading: true,
+      };
+
+    case POST_STATUS_CHANGES_REQUEST_SUCCESS:
+      return {
+        ...state,
+        items: action.postStatusChanges.map(postStatusChange => ({
+          postStatusId: postStatusChange.post_status_id,
+          userFullName: postStatusChange.user_full_name,
+          userEmail: postStatusChange.user_email,
+          updatedAt: postStatusChange.updated_at,
+        })),
+        areLoading: false,
+        error: '',
+      };
+
+    case POST_STATUS_CHANGES_REQUEST_FAILURE:
+      return {
+        ...state,
+        areLoading: false,
+        error: action.error,
+      };
+
+    case POST_STATUS_CHANGE_SUBMITTED:
+      return {
+        ...state,
+        items: [action.postStatusChange, ...state.items],
+      };
+
+    default:
+      return state;
+  }
+}
+
+export default postStatusChangesReducer;