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/saved_search.rb

@@ -130,10 +130,6 @@ class SavedSearch < ApplicationRecord
   end
 
   def disable_labels=(value)
-    CurrentUser.update(disable_categorized_saved_searches: true) if value == "1"
-  end
-
-  def disable_labels=(value)
-    CurrentUser.update(disable_categorized_saved_searches: true) if value == "1"
+    CurrentUser.update(disable_categorized_saved_searches: true) if value.to_s.truthy?
   end
 end