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/templates/manifest.gjson b/templates/manifest.gjson
index c8d2ffb..5aabcf1 100644
--- a/templates/manifest.gjson
+++ b/templates/manifest.gjson
@@ -102,6 +102,7 @@
         "css/ui_spacing/twbasic.css",
         "css/thread_page_design_warning.css",
         "css/thread_toolbar.css",
+        "css/flatten_threads.css",
 
         "communityConsoleMain.bundle.js.map",
         "communityConsoleStart.bundle.js.map",