Naive API endoint for getting survey statistics

Use case: we want to provide basic overview information for any given survey. Many surveys are too large to efficiently do this in the browser. 

This adds an endpoint at `api/surveys/:id/stats` that returns counts for each answer, like so:

    {
      survey: {
        dumping: {
          yes: 8,
          no: 9
        },
        fire-damage: {
          yes: 6
        },
        condition: {
          good: 2,
          poor: 3,
          fair: 1,
          excellent: 3
        },
        ...
      }
    }

Known deficiencies: 
- No caching. The data is recalculated on each request even if it hasn't changed
- Doesn't include stats on response times, users
- Data format may not be the best, depending on how we decide to consume the data (goal: no data transformation necessary on the client) 
- Data could include human-readable question text, so we don't have to join