Improve text chat

[bcpierce00]

With just a couple of improvements, it seems we should be able to make the text chat just as good as Sli.do for people to ask questions during talks.

E.g.,allow

• [ ] marking a post as a question and
• [ ] filtering the chat to show only questions.

(The former, especially, will be really helpful for replacing Slido!)

Jon's version: We should already have a chat channel for each paper, so add threading for Q&A. We implement that as JSON metadata on each message. Add upvoting and emoji reacts to messages (might be tricky to implement - need to worry about concurrency, can't just have increments on Twilio metadata)

Update: The way to avoid concurrency issues is simply to make each reaction or emoji be a new message (with an attribute that says which message it is reacting to).

[bcpierce00]

This may be superseded by current work on adding reactions to chat posts. Check later.

[mattmccutchen]

+1 to threaded chat. (Further +1s to this particular point can be added as GitHub reactions to this comment.)