Improve support for poll widgets

[neiljp]

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 Polls: Show who votes for each option #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)

[sahith-ch]

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[AngathG]

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[sahith-ch]

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Hello @sahith-ch, it looks like you've currently claimed 1 issue in this repository. We encourage new contributors to focus their efforts on at most 1 issue at a time, so please complete your work on your other claimed issues before trying to claim this issue again.

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[sahith-ch]

Hey, I've pushed all changes for that issue.
should i wait until its merger?

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[1shashwatharsh1]

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