String: add truthy? & falsy? core extensions.

* Add `truthy?` and `falsy?` core extensions to String.

* Use `truthy?` and `falsy?` to replace ad-hoc parsing of boolean
  parameters in various places.

app/models/upload.rb

@@ -426,9 +426,9 @@ class Upload < ApplicationRecord
         q = q.attribute_matches(:post_id, params[:post_id])
       end
 
-      if params[:has_post] == "yes"
+      if params[:has_post].to_s.truthy?
         q = q.where.not(post_id: nil)
-      elsif params[:has_post] == "no"
+      elsif params[:has_post].to_s.falsy?
         q = q.where(post_id: nil)
       end
 
@@ -490,10 +490,10 @@ class Upload < ApplicationRecord
   end
 
   def upload_as_pending?
-    as_pending == "1"
+    as_pending.to_s.truthy?
   end
 
   def include_artist_commentary?
-    include_artist_commentary == "1"
+    include_artist_commentary.to_s.truthy?
   end
 end