修复了assets中的若干问题

多余空行，缺漏分号，以及
```
WARNING on line 18 of /Users/jasl/workspace/ruby/19wu/app/assets/stylesheets/modules/_topic.css.scss: ".topic-item-title" failed to @extend ".media-heading".
  The selector ".media-heading" was not found.
  This will be an error in future releases of Sass.
  Use "@extend .media-heading !optional" if the extend should be able to fail.
```