Add "Allow Support votes" and "Allow Oppose votes" in the Add Election and Edit Election menus

[mitchdowney]

Some schools may not want to allow "oppose" votes on candidates. Although it is essential to civic discourse that dissenting voices can be clearly heard, I think we should allow election moderators the ability to disable vote types if they want.

These options would appear at the bottom of the Add Election and Edit Election menus, below the Election Description input. Both of these boxes would be checked by default, so the default mode of the EveryVote app is to allow both Support and Oppose votes.