Poll Block: add option to view results

When you use the Poll Block there is no way to view the results unless you submit the form.

### Steps to reproduce
1. Start a new post/page
2. Add the Poll Block
3. Save and check the page

### What I expected
I expected to see a link to `View Results` before submitting the poll as I see on the actual poll link (https://poll.fm/10642169) or when using the shortcode:

This could be an option in the block itself: **show/hide view results** link before submitting it.

### What happened instead

The “View Results” link isn’t there:

My test: https://eduardoistesting.blog/2020/10/30/testing-polls/

### Context / Source
#dogfooding

Start with the Poll Block by reproducing the three steps in the issue and compare its behavior with the linked poll and shortcode example. Trace where the block renders its controls and determine how an optional “View Results” link should be exposed. Done means users can show or hide that link before submitting the poll.

Written by the indexing model from the issue text.