Integrate the functionality provided by the addrelease plugin into Picard itself. One reason this was originally a plugin was because of the hacky way this worked (directly passing form parameters via query string). But the MB server's release editor seeding  allows to cleanly implement this.
it also allows us to fix
PICARD-2202 by serving the add release form (which is necessary to do the POST call) via Picard's built-in webserver.