ReplyTo option should be exposed through Setting API

Our customers have been complaining that CiviCRM used to allow you to send an email from one contact and for replies to be forwarded to a different contact.

I can see quite a bit of evidence that the civicrm_mailing.replyto_email field was actually used to store this data & likely works if populated.

However, the field is not exposed in the front end form - some of the code that 'would' expose it has been carried right through to the new angular forms. But the config variable that stored 'let's use it' has not been migrated to a setting and cannot be set from what I can tell. It is not declared in the setting metadata & I can't find it in an admin form.

This section of angular code (mailing.html)

<div crm-ui-field="subform.replyTo" crm-title="ts('Reply-To')" ng-show="crmMailingConst.enableReplyTo">
<div ng-controller="EmailAddrCtrl">
<select
crm-ui-id="subform.replyTo"
name="replyTo"
ui-jq="select2"
ui-options="

{dropdownAutoWidth : true, allowClear: true, placeholder: ts('Email address')}

"
ng-model="mailing.replyto_email"
ng-options="frm.label as frm.label for frm in crmFromAddresses.getAll() | filter:

{is_active:1}

| orderBy:'weight'"
>
<option value=""></option>
</select>
</div>

depends on the setting which is set in CRM_Mailing_Info to be equal to

'enableReplyTo' => isset($config->replyTo) ? $config->replyTo : 0,

$config->replyTo is not, however, settable to 1 from what I can see