When a project is redirected to a new project, Rails attempts to
construct the proper URL in `url_for` by finding the first route that
matches the given parameters. In this case, `namespace_id`,
`project_id`, `id` matched the `/designs` route first, so it used that.

This happens because the Rails `resources` block defines all the
standard #index, #show, #update, etc. routes after the block runs.

To ensure that `/issues/:id` is used to display an issue, we need to use
a separate action for `/issues/:id/designs`.

Closes https://gitlab.com/gitlab-org/gitlab/issues/31357