Migrate vote detail screens to the Pro Publica Congress API

[konklone]

This doesn't completely migrate vote data to the Pro Publica API, but it's a lot -- it fully migrates the vote details screen and the parsing of roll call data from the PP API.

I've verified that this properly handles Speaker of the House votes (which is informatically anomalous, since votes are names and not Yea/Nay, but can still contain Not Voting and Present).

Remaining voting-related issues (for a future PR):

• Recent vote lists/notifications - one complication here is that the PP API doesn't support "recent votes" in a paginated way, but https://github.com/propublica/congress-api-docs/issues/111 suggests it may be on its way.
• Votes by member - one complication used to be that the PP API wouldn't list the result or totals on this endpoint, but https://github.com/propublica/congress-api-docs/issues/103 suggests that this has been fixed.

[konklone]

cc @dwillis in case you're interested