Support zforms-based widgets (eg. trivia bot)

[neiljp]

A draft approach to this has been worked on in #287, but this was some time ago and was not developed further.

Once the interactivity features of polls and todo widgets are implemented, how to handle this for the zform case will likely be easier to understand.

Polls and todo widgets appear currently much more popular than zforms widgets, so this is not marked as help-wanted until the rest of the outstanding widgets work is complete.

[sauravk58]

@zulipbot claim

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