Adds an optional title attribute to polls. The rationale for this addition is that polls themselves didn't contain context/question and relied on post body to explain them. That context wasn't always obvious (e.g. when there are multiple polls in a single post) or available (e.g. when you display the poll breakdown - you see the answers, but not the question)
As a side note, here's a word on how the poll plugin works:
> We have a markdown poll renderer, which we use in the builder UI and the composer preview, but… when you submit a post, raw markdown is cooked into html (twice), then we extract data from the generated html and save it to the database. When it's render time, we first display the cooked html poll, and then extract some data from that html, get the data from the post's JSON (and identify that poll using the extracted html stuff) to then render the poll using widgets and the JSON data.
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
* Extract validation logic into a service object.
* Extract logic for updating polls custom fields into a service object.
* Use `strip_heredoc` instead.
* FIX: Polls do not update when configuration has been changed.
