This commit adds `durable=True` to the outermost db transactions
created in the following:
* confirm_email_change
* handle_upload_pre_finish_hook
* deliver_scheduled_emails
* restore_data_from_archive
* do_change_realm_subdomain
* do_create_realm
* do_deactivate_realm
* do_reactivate_realm
* do_delete_user
* do_delete_user_preserving_messages
* create_stripe_customer
* process_initial_upgrade
* do_update_plan
* request_sponsorship
* upload_message_attachment
* register_remote_server
* do_soft_deactivate_users
* maybe_send_batched_emails
It helps to avoid creating unintended savepoints in the future.
This is as a part of our plan to explicitly mark all the
transaction.atomic calls with either 'savepoint=False' or
'durable=True' as required.
* 'savepoint=True' is used in special cases.
