This required properly plumbing the guardian into the serializer.
Notably, the default state in the client was not changed - if you haven't voted in
the poll, you need to click the button to view the results instead of the results
being immediately visible on page load.
Implements https://meta.discourse.org/t/-/138108
The bug was mentioned here: https://meta.discourse.org/t/poll-name/136436
I added specs to cover existing behaviour and refactored can_see_results method.
Guard condition should apply only if the poll result setting is set to `staff_only`.
In other cases, user can see results when the poll result setting is set to `always` or user voted or poll is closed.
