Feature/post&tag

[r26zhao]

这个pr主要做了以下修改:

1. 去掉了postdetailserializer里的get_replies方法，因为现在replies通过 /posts/{id}/replies/获取
2. 优化了postdetailserializer里的get_participants_count方法，之前写的太复杂了>-<
3. 还有indexpostserializer里的 get_latest_reply_time
4. postviewset 里的 popular action， 原来我写的只返回前10条 --> queryset[: 10]， 现在queryset是48小时内所有热门帖子，然后paginator的page_size 改成了10

另外 select_related 和 prefetch_related 我没怎么用过，不知道如何优化 = =

[r26zhao]

哎= =？ 怎么25号之前的commit还在。。

[coveralls]

Pull Request Test Coverage Report for Build 66

• 25 of 26 (96.15%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 83.001%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
posts/serializers.py	3	4	75.0%

Totals
Change from base Build 64:	0.7%
Covered Lines:	1665
Relevant Lines:	2006

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💛 - Coveralls