Add 'Start a thread' option to the chat context menu

Closes #2186 (closed)

• introduce threaded-conversations feature toggle
• add stats to replying to both clicking on the user name and selecting Reply in the message context menu
  • clicking on the user name: chatItem.mentionUser
  • clicking on Reply in the message context menu: chatItem.actions.reply
• under the feature toggle remove Reply option from the message context menu
• under the feature toggle add Start a thread option to the message context menu

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 Reply still works

4. Without feature toggle clicking Reply and username send a stat (chatItem.actions.reply and chatItem.mentionUser respectively) to http://localhost:5000/api/private/statsc (visible in the network panel)

5. With feature toggle there is a Start a thread replacing the Reply option