routes: replace hardcoded routes in models with route helpers.

Add a Routes module that gives models access to route helpers outside of
views, and use it to replace various hardcoded routes.

app/models/bulk_update_request.rb

@@ -89,7 +89,7 @@ class BulkUpdateRequest < ApplicationRecord
     end
 
     def bulk_update_request_link
-      %{"bulk update request ##{id}":/bulk_update_requests?search%5Bid%5D=#{id}}
+      %{"bulk update request ##{id}":#{Routes.bulk_update_requests_path(search: { id: id })}}
     end
   end