Flatten threads: add UI components to messages

- A quote is added to messages to show which was the previous message in
  the reply chain.
- A "reply" button is added to messages to allow users to compose a
  comment which is added at the end of the reply chain.
- A bug is fixed in calculating the parent reply ID in the extra info
  object. Now parent reply means the first message in a reply chain,
  while previous reply means the previous message in the reply chain.

Bug: twpowertools:153
Change-Id: I699507ade52e80287dd634e61f835d53af6a904d
diff --git a/src/injections/litComponentsInject.js b/src/injections/litComponentsInject.js
index f4d59bd..9506c8c 100644
--- a/src/injections/litComponentsInject.js
+++ b/src/injections/litComponentsInject.js
@@ -4,6 +4,7 @@
 // because `window.customElements` doesn't exist in content scripts.
 import '../contentScripts/communityConsole/workflows/components/index.js';
 import '../contentScripts/communityConsole/threadToolbar/components/index.js';
+import '../contentScripts/communityConsole/flattenThreads/components/index.js';
 
 import {injectStylesheet} from '../common/contentScriptsUtils.js';