Possibility to disable sign up / registering

It should be possible to disable signing up / registering to the platform. This should be a configuration flag under the Decidim initializer. What this would affect: 1. Remove the "registerable" module from Decidim::User model 2. Hide the "Sign up" link from the header as well as the sign in page (latter should be automatic in case 1. is implemented) 3. Disable the "sign_up" related routes, i.e. not to mount them at all The main reason for this is that some organizations may want to use their own sign in service which also handles sign ups. Another reason is that in some instances, signing up should not be possible at all as described in: https://meta.decidim.org/processes/roadmap/f/122/proposals/13132