Fix /wp-admin URLs

There are several places in the PRO plugin where URLs are hard-coded with `/wp-admin` path. Including in the installation procedure, ie. when generating the default affiliate and affiliate registration pages. And right now, I found more on the Creatives page under “View Creatives” and “Add New Creatives” buttons.

This is incompatible on WordPress websites which use a custom `/wp-admin` folder path, and results in the links/buttons going to 404 pages.

WordPress admin URLs should be generated using `admin_url(‘edit.php?post_type=wcu-creatives’);`, and shouldn’t assume a path of `/wp-admin`.