Polls: Show who votes for each option

[rsashank]

What does this PR do, and why?

Add support to view polls

External discussion & connections

• [x] Discussed in #zulip-terminal in Polls - Widget Support #T986 #T1477
• [ ] Fully fixes #
• [x] Partially fixes issue #986
• [x] Builds upon previous unmerged work in PR #1549 (first 2 commits)
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [ ] Merge will enable work on #

How did you test this?

• [x] Manually - Behavioral changes
• [x] Manually - Visual changes
• [x] Adapting existing automated tests
• [x] Adding automated tests for new behavior (or missing tests)
• [x] Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• [x] It is a minimal coherent idea
• [x] It has a commit summary following the documented style (title & body)
• [x] It has a commit summary describing the motivation and reasoning for the change
• [x] It individually passes linting and tests
• [x] It contains test additions for any new behavior
• [x] It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes

Before	After	Webapp

[neiljp]

@rsashank I've merged the first two commits and pushed back here, both with minor adjustments.

I'm marking this as a completion candidate for now, since there were a few outstanding concerns from what I could tell. #1573 is intended to track further poll widget work.

[neiljp]

Updated PR name to reflect remaining commits.