posts: micro-optimize allocations during thumbnail generation.

Do a few micro-optimizations to reduce the number of memory allocations
during thumbnail generation.

This commit, combined with freezing string literals in a7dc05 and
67b961, reduces the number of allocations on the front page from 180,000
to 150,000, and the number of retained objects from 8,000 to 4,000.

app/components/post_preview_component.rb

@@ -123,6 +123,6 @@ class PostPreviewComponent < ApplicationComponent
   end
 
   def has_sound?
-    post.has_tag?("sound")
+    is_animated? && post.has_tag?("sound")
   end
 end