TRACKING: Increase support for widgets (**poll**, todo, trivia, ...)

[neiljp]

This needs careful handling since it is a fairly underdeveloped feature in the server which may change. However it has not changed significantly for some time and /poll is now used quite often. On that basis it would be good to be able to:

• View polls which are in messages
• Have polls update as users 'vote'
• Respond to existing options
• Add new options

These are in order of extra features - viewing a poll would be a good start, ideally up to date, etc.

Note that there are outstanding issues for elements like /poll which don't support markdown, and that the message itself shouldn't be edited (zulip/zulip#17156).

An existing approach applied to the trivia bot is in #287 which could provide a starting point for that particular case (zforms), though it would be good to focus specifically on polls first as they are more common.

For server reference material, see https://zulip.readthedocs.io/en/latest/subsystems/widgets.html

Once we have infrastructure for /poll then other similar features should be easier to add.

NOTE re slash commands in general:
Note that we already actively support one "command" feature, ie /me, and there are more specific commands we could add (shortcuts to settings, setting themes, etc) like in the webapp. However, there has been discussion on splitting no-result commands and commands generating messages into separate UI elements, and we likely don't need slash commands as much in the terminal, at least in the short term.