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/bulk_update_request.rb

@@ -205,10 +205,6 @@ class BulkUpdateRequest < ApplicationRecord
   end
 
   def skip_secondary_validations=(v)
-    if v == "1" or v == true
-      @skip_secondary_validations = true
-    else
-      @skip_secondary_validations = false
-    end
+    @skip_secondary_validations = v.to_s.truthy?
   end
 end