Explicitly link themes to avoid link_tree issues

`link_tree` seems to be failing when the server is restarted.
Given we're just providing two themes we can list them individually.

backend/app/assets/config/solidus_backend_manifest.js

@@ -5,4 +5,5 @@
 
 //= link solidus_admin/select2_locales
 
-//= link_directory ../stylesheets/spree/backend/themes .css
+//= link spree/backend/themes/classic.css
+//= link spree/backend/themes/solidus_admin.css