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.
The migration script is not idempotent due to database constrains on the
poll related objects, namely:
polls: index_polls_on_post_id_and_name (post_id,name) UNIQUE
poll_options: index_poll_options_on_poll_id_and_digest (poll_id,digest) UNIQUE
poll_votes: index_poll_votes_on_poll_id_and_poll_option_id_and_user_id (poll_id,poll_option_id,user_id) UNIQUE
This change skips a particular poll migration if it's already found on
the db.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
