Add onboarding state selector for self-hosted signup (#251)

* Add onboarding modes to self-hosted signup

* Style form consistently

* Configure ONBOARDING_STATE via ENV

app/controllers/registrations_controller.rb

@@ -3,6 +3,7 @@ class RegistrationsController < ApplicationController
 
   layout "auth"
 
+  before_action :ensure_signup_open, if: :self_hosted?
   before_action :set_user, only: :create
   before_action :set_invitation
   before_action :claim_invite_code, only: :create, if: :invite_code_required?
@@ -79,4 +80,10 @@ class RegistrationsController < ApplicationController
         render :new, status: :unprocessable_entity
       end
     end
+
+    def ensure_signup_open
+      return unless Setting.onboarding_state == "closed"
+
+      redirect_to new_session_path, alert: t("registrations.closed")
+    end
 end