assets: migrate stylesheets from Sass to CSS

GitHub: ref rangubaGH-35

We will remove Sass because node-sass which is dependency has already been deprecated.
That's why we migrate it from Sass to CSS.

app/assets/stylesheets/search.scss → app/assets/stylesheets/search.css

@@ -1,6 +1,7 @@
-// Place all the styles related to the posts controller here.
-// They will automatically be included in application.css.
-// You can use Sass (SCSS) here: http://sass-lang.com/
+/*
+ * Place all the styles related to the posts controller here.
+ * They will automatically be included in application.css.
+ */
 
 /* layout */
 .content {