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/Post/ActionBox.tsx

@@ -0,0 +1,33 @@
+import * as React from 'react';
+import Button from '../shared/Button';
+
+import { BoxTitleText, SmallMutedText } from '../shared/CustomTexts';
+
+interface Props {
+  followed: boolean;
+  submitFollow(): void;
+
+  isLoggedIn: boolean;
+}
+
+const ActionBox = ({followed, submitFollow, isLoggedIn}: Props) => (
+  <div className="actionBoxContainer">
+    <div className="actionBoxFollow">
+      <BoxTitleText>Actions</BoxTitleText>
+      <br />
+      <Button onClick={isLoggedIn ? submitFollow : () => location.href = '/users/sign_in'} outline>
+        { followed ? 'Unfollow post' : 'Follow post' }
+      </Button>
+      <br />
+      <SmallMutedText>
+        { followed ?
+          'you\'re receiving notifications about new updates on this post'
+          :
+          'you won\'t receive notifications about this post'
+        }
+      </SmallMutedText>
+    </div>
+  </div>
+);
+
+export default ActionBox;