Use namespace as symbol in polymorphic_path calls

Our `namespace` helper returns a string. However, Rails version 5.2.4.6
doesn't allow strings as arguments to polymorphic_path [1]

Since returning a symbol in our `namespace` helper would break other
places in the application, we're converting it to a symbol in the
methods calling `polymorphic_path`.

[1] https://github.com/advisories/GHSA-hjg4-8q5f-x6fm

config/initializers/routes_hierarchy.rb

@@ -30,9 +30,9 @@ module ActionDispatch::Routing::UrlFor
       resolve = resolve_for(resource)
       resolve_options = resolve.pop
 
-      polymorphic_path([namespace, *resolve], options.merge(resolve_options))
+      polymorphic_path([namespace.to_sym, *resolve], options.merge(resolve_options))
     else
-      polymorphic_path([namespace, *resource_hierarchy_for(resource)], options)
+      polymorphic_path([namespace.to_sym, *resource_hierarchy_for(resource)], options)
     end
   end