Ensure `project/issues#designs` is correctly located in EE code

This is a follow up to !16638 (merged)

In this MR, we resolved the redirect issue by adding a new action in the Projects::IssuesController, namely #designs. ~"design management" however is a GitLab Premium feature, so we should really not have this action in the vanilla controller. This issue is to discuss moving the action and any other aspects (including the route definition) to EE only files.

/cc @stanhu