Fix unqualified column references.

Fix various places to avoid unqualified column references to prevent any
potential ambiguous column errors.

app/models/post.rb

@@ -983,11 +983,11 @@ class Post < ApplicationRecord
     end
 
     def random_up(key)
-      where("md5 < ?", key).reorder("md5 desc").first
+      where("md5 < ?", key).reorder(md5: :desc).first
     end
 
     def random_down(key)
-      where("md5 >= ?", key).reorder("md5 asc").first
+      where("md5 >= ?", key).reorder(md5: :asc).first
     end
 
     def sample(query, sample_size)