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.

config/initializers/core_extensions.rb

@@ -23,6 +23,14 @@ module Danbooru
       def to_escaped_for_tsquery
         "'#{gsub(/\0/, '').gsub(/'/, '\0\0').gsub(/\\/, '\0\0\0\0')}'"
       end
+
+      def truthy?
+        self.match?(/\A(true|t|yes|y|on|1)\z/i)
+      end
+
+      def falsy?
+        self.match?(/\A(false|f|no|n|off|0)\z/i)
+      end
     end
   end
 end