Add Events to API

[connorshea]

Part of #735.

This adds events to the API as well as an activity endpoint that you can use to list events either in the 'following' scope or 'global' scope.

query testActivity {
  activity(feedType: GLOBAL) {
    user {
      username
    }
    eventCategory
  }
}

You can also list events for a given user. Because Events have a polymorphic relationship with the 'eventable', you have to do some special stuff to get data from them.

query ($id: ID!) {
  user(id: $id) {
    avatarUrl
    id
    username
    events {
      id
      user {
        username
      }
      eventCategory
      eventable {
        ... on User {
          __typename
          username
        }
        ... on Relationship {
          __typename
          follower {
            username
          }
          followed {
            username
          }
        }
        ... on GamePurchase {
          __typename
          game {
            name
          }
        }
        ... on FavoriteGame {
          __typename
          game {
            name
          }
        }
      }
    }
  }
}