Improve support for poll widgets

Initial work in #1551 by @rsashank provided

• parsing of poll data
• rendering into a styled message
• updating of the styled message upon changes in the server (events)

This issue is intended to split out poll-related follow-up work which was originally tracked in #986.

Outstanding elements include

• Showing who has voted for each poll option (currently the remaining work in #1551)
• Enable responding to polls, ie. toggling options
• Allow editing the question in your own polls
• Allow adding options to polls
• Limiting the editing of polls in the client (widget messages (content) may not be edited)