Replace hardcoded domains with primary domain setting

app/controllers/signup_controller.rb

@@ -88,7 +88,7 @@ class SignupController < ApplicationController
     if session[:new_user].present?
       @user = User.new(session[:new_user])
     else
-      @user = User.new(ou: "kosmos.org")
+      @user = User.new(ou: Setting.primary_domain)
     end
   end
 
@@ -98,7 +98,7 @@ class SignupController < ApplicationController
 
     CreateAccount.call(
       username: @user.cn,
-      domain: "kosmos.org",
+      domain: Setting.primary_domain,
       email: @user.email,
       password: @user.password,
       invitation: @invitation