Fix classroom session refreshing on chat message

[sumanyu]

Major changes

• Added two types of messages normal and alert. alert messages example: user_name has left the session.
• Fixed classroom subscription logic where the mongodb query wasn't working properly. After hours of trial and error I figured out why.
• Prevent entire page from re-rendering when a person types something

  Fix later

• We've implemented chat such that the entire chat messages will re-render. Thus, chat becomes slow later. It shouldn't but, it is. We'll need to look into this later.

[davinwong]

approved. we are doing well with these well sized pull requests that make a lot of sense.