Members only posts on activity feed

Members only posts (aka "login required" posts), see #57.

Current:
- display using same rule as other posts on blog page (full post), display only to logged in users (keep current implementation)
- members only posts are not displayed on home activity feed

Expected:
- display in short form (Title and post summary) on home page activity feed only to logged in users

首先查看 issue #57，以及现有的仅限成员文章的博客页面处理逻辑。然后定位主页活动 feed 的实现，并确定登录用户可见性以及短格式标题/摘要渲染的处理方式；完成的标准是符合条件的文章仅对登录用户显示在其中，同时不改变当前博客页面的行为。

由索引模型根据 Issue 内容生成。