posts: fix thumbnails on comment and modqueue pages.

Fix thumbnails being fit-to-width instead of a fixed size on the
/comments and /modqueue pages, which caused the columns to be misaligned.

app/helpers/components_helper.rb

@@ -1,6 +1,6 @@
 module ComponentsHelper
-  def post_preview(post, **options)
-    render PostPreviewComponent.new(post: post, **options)
+  def post_preview(post, fit: :fixed, **options)
+    render PostPreviewComponent.new(post: post, fit: fit, **options)
   end
 
   # Render a set of posts as thumbnail gallery.