Improve `Other` collating for less-competitive races

[mileswwatkins]

If there is only one major-party candidate in the race, then the second candidate should not be collated into Other; that creates weird key-results big-boards like this, when there's clearly a third-party candidate with a good deal of votes:

cc https://github.com/nprapps/elections18-graphics/issues/56

[mileswwatkins]

This is a table of how this ought to break down:

Standard competitive races:
- D,R,O,O,... -> D,R
- D,R -> D,R
- D,O -> D,O
- R,O -> R,O

Less likely, relatively uncontested races:
- D,O,O -> D,O
- R,O,O -> R,O

This won't happen for a top-of-ticket seat, but it's handled:
- O,O,O -> O,O

Top-two general election (eg, California or Louisiana):
- D,D -> D,D
- R,R -> R,R

Uncontested races:
- D -> D
- R -> R
- O -> O

[mileswwatkins]

Bam