To-do lists

[gnprice]

I.e., this Zulip feature:
https://zulip.com/help/collaborative-to-do-lists

At the API level, this will re-use a lot of the infrastructure we build for polls, #164.

This is a post-launch feature because zulip-mobile doesn't have it. I believe this Zulip feature is also a lot less commonly used than polls.