topic with favorite/follow status

通过api获取topic时, 为其添加`has_favorited`与`has_followed`两条属性. 用于判断当前用户是否已经`喜欢`或`收藏`此topic. 
若用户未登陆, 此topic的`has_favorited`与`has_followed`属性皆为`false`.
若用户已登陆, 则判断当前用户是否喜欢/收藏此topic, 并返回`true`或`false`.
具体的判断代码copy自`TopicsController#check_current_user_status_for_topic`
*此功能用于丰富api使用端的用户体验*.
测试代码已通过, 如下:
```ruby
it "should get favorite/follow status with current_user" do
      t = Factory(:topic, :title => "Have I favorited this topic?")
      get "/api/v2/topics/#{t.id}.json"
      expect(response.status).to eq(200)
      json = JSON.parse(response.body)
      puts json["has_followed"]
      expect(json["has_followed"]).to eq(false)
      expect(json["has_favorited"]).to eq(false)
      u = Factory(:user).tap { |u| u.update_private_token }
      #like it
      post "/api/v2/topics/#{t.id}/favorite.json", :token => u.private_token

      get "/api/v2/topics/#{t.id}.json", :token => u.private_token
      expect(response.status).to eq(200)
      json = JSON.parse(response.body)
      expect(json["has_favorited"]).to eq(true)
      expect(json["has_followed"]).to  eq(false)

      #unlike it
      post "/api/v2/topics/#{t.id}/favorite.json", :token => u.private_token, :type => 'unfavorite'

      #follow it
      post "/api/v2/topics/#{t.id}/follow.json", :token => u.private_token

      get "/api/v2/topics/#{t.id}.json", :token => u.private_token
      expect(response.status).to eq(200)
      json = JSON.parse(response.body)
      expect(json["has_favorited"]).to eq(false)
      expect(json['has_followed']).to  eq(true)

      get "/api/v2/topics/#{t.id}.json"
      expect(response.status).to eq(200)
      json = JSON.parse(response.body)
      expect(json["has_favorited"]).to eq(false)
      expect(json['has_followed']).to  eq(false)
    end

    it "should work when id record found" do
      get "/api/topics/-1.json"
      expect(response.status).to eq(404)
    end
  end
```