Add and apply Rails/WhereNot rubocop rule

We simplify the code a little bit, and make it more consistent since we
were already using `where.not` in most places.

app/models/verification/management/managed_user.rb

@@ -2,7 +2,7 @@ class Verification::Management::ManagedUser
   include ActiveModel::Model
 
   def self.find(document_type, document_number)
-    User.where("document_number is not null").
+    User.where.not(document_number: nil).
     find_or_initialize_by(document_type:   document_type,
                           document_number: document_number)
   end