修改账户系统以及部分用户权限写法

增加部分测试和注释，完善国际化

problem/serizalizers.py

@@ -1,9 +1,8 @@
 # coding=utf-8
-import json
-
 from rest_framework import serializers
 
 from account.models import User
+from utils.serializers import JSONField
 from .models import Problem, ProblemTag
 
 
@@ -12,11 +11,6 @@ class ProblemSampleSerializer(serializers.ListField):
     output = serializers.CharField(max_length=3000)
 
 
-class JSONField(serializers.Field):
-    def to_representation(self, value):
-        return json.loads(value)
-
-
 class CreateProblemSerializer(serializers.Serializer):
     title = serializers.CharField(max_length=50)
     description = serializers.CharField(max_length=10000)