#1724 - Backward-compatible handling of 'contribution_invoice_settings.invoicing'

In the old setting `contribution_invoice_settings`, the `invoicing`
property (aka "CiviContribute Component Settings" => "Enable Tax and
Invoicing") could take one of two values:

1. If enabled, it's an array with trueish subkey: `['invoicing'=>1]`
2. If disabled, it's a numeric zero.

This encoding is weird, and `contribution_invoice_settings` is generally deprecated
and has been replaced by a virtual rendition which needs to be consistent with the
old representation -- so that a couple-dozen things continue working.

For testing, it helped me to try these commands at different points:

```
cv ev 'return CRM_Invoicing_Utils::isInvoicingEnabled();'
cv api setting.get return=invoicing
cv api setting.get return=contribution_invoice_settings

select * from civicrm_setting where name like 'contrib%' or name like 'invoic%';
```

Civi/Core/SettingsBag.php

@@ -287,6 +287,9 @@ class SettingsBag {
     if ($key === 'contribution_invoice_settings') {
       foreach (SettingsBag::getContributionInvoiceSettingKeys() as $possibleKeyName => $settingName) {
         $keyValue = $value[$possibleKeyName] ?? '';
+        if ($possibleKeyName === 'invoicing' && is_array($keyValue)) {
+          $keyValue = $keyValue['invoicing'];
+        }
         $this->set($settingName, $keyValue);
       }
       return TRUE;
@@ -302,7 +305,15 @@ class SettingsBag {
   public function computeVirtual() {
     $contributionSettings = [];
     foreach (SettingsBag::getContributionInvoiceSettingKeys() as $keyName => $settingName) {
-      $contributionSettings[$keyName] = $this->get($settingName);
+      switch ($keyName) {
+        case 'invoicing':
+          $contributionSettings[$keyName] = $this->get($settingName) ? [$keyName => 1] : 0;
+          break;
+
+        default:
+          $contributionSettings[$keyName] = $this->get($settingName);
+          break;
+      }
     }
     return ['contribution_invoice_settings' => $contributionSettings];
   }