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/package.json b/package.json
index 5771ded..5e9a073 100644
--- a/package.json
+++ b/package.json
@@ -46,6 +46,7 @@
     "@material/tooltip": "^12.0.0",
     "@material/web": "^0.1.0-alpha.0",
     "async-mutex": "^0.3.2",
+    "dompurify": "^2.4.1",
     "google-protobuf": "^3.19.3",
     "grpc-web": "^1.2.1",
     "idb": "^6.1.2",