Update RCV tabulator to handle invalid ballots

[ArendPeter]

In the video on #723, there were 393 ballots, but only 244 registered in the results. That's because many voters didn't give a first rank. There's skip rank rules to handle this, and this also needs to be an election setting

[ArendPeter]

From #742

Filling in ranks sequentially will always be the "correct" way to fill out a ballot, but that said every jurisdiction needs to have a fallback plan for interpreting ballots that don't properly follow the rules. San Francisco will ignore any number of skipped columns, whereas NYC and Alaska will exhaust the ballot when there's more than 2 skipped columns.

Since we're adding support for uploading ballots from real world RCV elections we do need to have a setting for this for the tabulator, but not the ballot UX