display configured batch size in form

The batch size was getting set on form submission, but continuing to
only display 100 in the form regardless of the configured value after
changing it.

src/Form/SettingsForm.php

@@ -155,7 +155,7 @@ class SettingsForm extends ConfigFormBase {
         '#description' => $this->t('
           <p>Set how many files will be processed at once when performing a batch / cron job</p>
         '),
-        '#default_value' => 100,
+        '#default_value' => $config->get(static::BATCH_SIZE) ?: 100,
       ],
     ];