polls: Edit the poll question

[gnprice]

As part of poll support (#164), we should support editing the poll question (aka its name).

This is something the poll creator can do after creating the poll. See https://zulip.com/help/create-a-poll .

This will probably come after #166 and #167, and certainly after #165. It corresponds to an open feature request in zulip-mobile:

• Allow editing poll question  zulip-mobile#4881