Add `stats` property to User graphql schema

To enable the following query:

``` js
query {
  getPlayerById($playerId) {
    stats {
      avgProjectCompleteness,
      avgProjectQuality,
      avgCultureContribution,
      avgLearningSupport,
      avgSupportStat,
      totalHoursWorked,
      totalProjectsReviewed,
      totalExperiencePoints,
    }
  }
}
```
