PERF: optimize homepage and topic performance (#11607)

These are a few small tweaks that slightly improve performance.

- we omitted 1 query from the post guardian which could cause an N+1
- cook_url has been sped up a bit
- url helper avoids re-creating sets for no reason

lib/file_helper.rb

@@ -14,15 +14,15 @@ class FileHelper
   end
 
   def self.is_supported_image?(filename)
-    (filename =~ supported_images_regexp).present?
+    filename.match?(supported_images_regexp)
   end
 
   def self.is_inline_image?(filename)
-    (filename =~ inline_images_regexp).present?
+    filename.match?(inline_images_regexp)
   end
 
   def self.is_supported_media?(filename)
-    (filename =~ supported_media_regexp).present?
+    filename.match?(supported_media_regexp)
   end
 
   class FakeIO
@@ -162,7 +162,10 @@ class FileHelper
   end
 
   def self.supported_media_regexp
-    media = supported_images | supported_audio | supported_video
-    @@supported_media_regexp ||= /\.(#{media.to_a.join("|")})$/i
+    @@supported_media_regexp ||=
+      begin
+        media = supported_images | supported_audio | supported_video
+        /\.(#{media.to_a.join("|")})$/i
+      end
   end
 end