Cater for polls that can have multiple votes per user.
This fixes an older UserMerge and migration which was intended to
de-duplicate poll votes but did not account for "multiple" type polls.
When merging users, polls may error out if the source and target users have both voted on the same poll before. 😢
There is no constraint on the `poll_votes` table either to support this. Ideally a composite primary key can be used `(poll_id, user_id)`, but alas there is no support yet, which is probably why it wasn't created in the first place.
This fix ensures that merging is successful by only keeping the target poll votes if duplicates exist.
This fix also runs a migration on older poll votes where failed merges would have caused a single user to have voted twice on a single poll. e.g. this weird edge case
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 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
