Show parent message in the thread message feed

Closes #2190 (closed)

• remove code that puts messages from initial render to Vuex store
• display parent message when opening thread message feed
• support KaTeX styles from Vue less loader
• update messageMap when changing room
• update messageMap when live-loading new messages / updating messages
• update messageMap with messages fetched during scrolling
• update messageMap when submitting/updating a new message
• make the thread-message-feed>chat-item style more compact
• format sent date and show it
• handle trimming of user and message info in small space
• dark theme
• make KaTeX fonts work in both the old less and the Vue styles

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. click on actions menu on a message in the main message feed

4. select "Start a thread"

5. see the parent message showing in the thread message feed

6. toggle dark theme and see parent message in the thread message feed

7. close the thread message feed - the right toolbar should show

8. open the thread message feed again and change room - TMF should be closed and the right toolbar opened

9. error message

   1. create a new message and start a thread from it
   2. delete the message in the main message feed
   3. scroll up to trigger collection synchronisation
   4. you should see an error message: Error: The message for this thread is unavailable. It was probably deleted.

10. test messages:

    • katex $$|\vec{A}|=\sqrt{A_x^2 + A_y^2 + A_z^2}.$$(1)
    • multiline code highlighting (using three `)