Add `group_owners_can_manage_default_branch_protection ` column

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

changelogs/unreleased/211944-provide-instance-level-setting-to-enable-or-disable-default-branch.yml

+---
+title: Add a database column to enable or disable group owners from changing the default
+  branch protection setting of a group
+merge_request: 29397
+author:
+type: added

db/migrate/20200413072059_add_group_owners_can_manage_default_branch_protection_to_application_settings.rb

+# frozen_string_literal: true
+
+class AddGroupOwnersCanManageDefaultBranchProtectionToApplicationSettings < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  disable_ddl_transaction!
+
+  def up
+    add_column_with_default(:application_settings,
+                            :group_owners_can_manage_default_branch_protection,
+                            :boolean,
+                            default: true)
+  end
+
+  def down
+    remove_column :application_settings, :group_owners_can_manage_default_branch_protection
+  end
+end

db/structure.sql

@@ -400,7 +400,8 @@ CREATE TABLE public.application_settings (
     seat_link_enabled boolean DEFAULT true NOT NULL,
     container_expiration_policies_enable_historic_entries boolean DEFAULT false NOT NULL,
     issues_create_limit integer DEFAULT 300 NOT NULL,
-    push_rule_id bigint
+    push_rule_id bigint,
+    group_owners_can_manage_default_branch_protection boolean DEFAULT true NOT NULL
 );
 
 CREATE SEQUENCE public.application_settings_id_seq
@@ -13198,6 +13199,7 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200408175424
 20200409211607
 20200410232012
+20200413072059
 20200414144547
 20200415160722
 20200415161021