Implement thumbs up/down (reactions)

[ericsoco]

@waldoj says thumbs up/down voting is "useless...but necessary". Not of any intrinsic/real value for determining quality of proposal, but great for promoting engagement: a casual user may not care enough to comment on a proposal, but if they get an email encouraging them to "stop by and give it a thumbs up" then there's a higher chance of getting at least a minimal level of engagement, from a broader user base.

GitHub reactions API

[ericsoco]

dupe of #23