add_forge_now: Redirect to main archive for mirror version of swh-web
As the Add Forge Now service will not be available for mirrors, redirect to the main archive in a new browser tab when clicking on the Add Forge Now link in the main menu, also ensure related URLs will not be defined by removing the swh.web.add_forge_now django application from the enabled applications list.
Also rework external Save Code Now integration by proceeding the same.
Related to #4748 (closed)