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/components/Comments/NewCommentUpdateSection.tsx

@@ -0,0 +1,33 @@
+import * as React from 'react';
+import { MutedText } from '../shared/CustomTexts';
+
+interface Props {
+  postUpdateFlagValue: boolean;
+  handlePostUpdateFlag(): void;
+}
+
+const NewCommentUpdateSection = ({
+  postUpdateFlagValue,
+  handlePostUpdateFlag,
+}: Props) => (
+  <div className="commentIsUpdateForm">
+    <div>
+      <input
+        id="isPostUpdateFlag"
+        type="checkbox"
+        onChange={handlePostUpdateFlag}
+        checked={postUpdateFlagValue || false}
+      />
+      &nbsp;
+      <label htmlFor="isPostUpdateFlag">Mark as post update</label>
+    </div>
+    {
+      postUpdateFlagValue ?
+        <MutedText>Users that follow this post will be notified</MutedText>
+      :
+        null
+    }
+  </div>
+);
+
+export default NewCommentUpdateSection;