core: simplify contest & homework logic

packages/ui-default/locales/zh.yaml

@@ -782,6 +782,7 @@ Testdata Config: 测试数据配置
 Testdata: 测试数据
 Text: 文本
 The 'default' role applies to ALL REGISTERED USER.: default 角色作用于<b>所有已注册用户</b>
+The contest is a flexible time contest. You need to complete the contest within a specified time after you attended.: 本场比赛采用灵活时间模式，你需要在参加后的指定时间内完成比赛。
 The homework's deadline is due but in extension. You can still submit for this problem but your score will be penalized.: 作业已超过截止时间，但仍在延期时间内。您递交题目将无法获得全部分数。
 The invitation code to enter to successfully join the domain. You can only use letters and numbers in the code and it should not be longer than 64 characters.: 加入此域的邀请码。您只能使用字母和数字，并且不能长于 64 个字符。
 The invitation code you provided is invalid.: 您提供的邀请码无效。