Add `require_admin_approval_after_user_signup ` column

This change adds
`require_admin_approval_after_user_signup `  column
to `application_settings` table

changelogs/unreleased/257881-migration-user-admin-approval-toggle.yml

+---
+title: Add a database column to enable or disable the setting that puts newly registered
+  users in a pending state, requiring admin approval for their activation
+merge_request: 43661
+author:
+type: added

db/migrate/20200929063159_add_require_admin_approval_after_user_signup_to_application_settings.rb

+# frozen_string_literal: true
+
+class AddRequireAdminApprovalAfterUserSignupToApplicationSettings < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def change
+    add_column :application_settings, :require_admin_approval_after_user_signup, :boolean, default: false, null: false
+  end
+end

db/schema_migrations/20200929063159

+9ef08404b964ccae3e12332340f16c6b8bb2bbdb2c04ba105fe1c0c7e6bf84f2
\ No newline at end of file

db/structure.sql

@@ -9270,6 +9270,7 @@ CREATE TABLE application_settings (
     gitpod_enabled boolean DEFAULT false NOT NULL,
     gitpod_url text DEFAULT 'https://gitpod.io/'::text,
     abuse_notification_email character varying,
+    require_admin_approval_after_user_signup boolean DEFAULT false NOT NULL,
     CONSTRAINT check_2dba05b802 CHECK ((char_length(gitpod_url) <= 255)),
     CONSTRAINT check_51700b31b5 CHECK ((char_length(default_branch_name) <= 255)),
     CONSTRAINT check_9c6c447a13 CHECK ((char_length(maintenance_mode_message) <= 255)),