webui: unitreport: prevent unnecessary escapes

before the report did contain lines like:
"fileset": "\u003Call file-based\u003E"

Now it will be stored in the valid and intended form:
"fileset": "<all file-based>"

Also "/" will no longer be escaped (like in "Debian GNU\/Linux").

webui/module/Director/src/Director/Controller/DirectorController.php

@@ -185,7 +185,8 @@ public function backupunitreportAction()
         $response->getHeaders()->addHeaderLine('Content-Disposition', 'attachment; filename="bareos-backup-unit-report.json"');
 
         if (isset($result)) {
-            $response->setContent(JSON::prettyPrint(JSON::encode($result)));
+            $json_backupunitreport = json_encode($result, JSON_UNESCAPED_SLASHES | JSON_PRETTY_PRINT);
+            $response->setContent($json_backupunitreport);
         }
 
         return $response;