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

app/views/admin/widget/cards/_form.html.erb

@@ -1,6 +1,6 @@
 <%= render "shared/globalize_locales", resource: card %>
 
-<%= translatable_form_for [namespace, card.cardable, card], url: local_assigns[:url] do |f| %>
+<%= translatable_form_for [namespace.to_sym, card.cardable, card], url: local_assigns[:url] do |f| %>
   <%= render "shared/errors", resource: card %>
 
   <div class="row">

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