Add thread message feed

Part of #2190 (closed)

• Make the right toolbar and thread message feed coexist together
• validate that one store can be used for two Vue instances
• wrap rendering of thread message feed with feature toggle
• Make the 'Start a thread' context menu open the thread message feed
• Add the 'Thread' header and X icon to close the thread message feed
• Dark theme

Will be implemented in another issue

• show header and parent message at the top of the
• show chat input view at the bottom of thread message feed
• make the chat input submit plain message

Testing strategy

1. Add the feature toggle, node ./scripts/utils/feature-toggle.js --name threaded-conversations --description "Enabling discussion in a thread #741"

2. Enable the feature toggle

   • Enable: http://localhost:5000/api_web/features/threaded-conversations/1
   • Disable: http://localhost:5000/api_web/features/threaded-conversations/0

3. Without feature toggle, there is still 'Reply' option in message context menu.

4. With feature toggle, click on 'Start a thread' option in message context menu and see the thread message feed open.

5. Click on the X icon and see the thread message feed being replaced with the right toolbar.