[files_external] fix config gui for all users

backport of 9e05781ec22c3f4e6b89b188332242b03088736e

apps/files_external/templates/settings.php

@@ -83,7 +83,7 @@
 							<select class="chzn-select"
 									multiple style="width:20em;"
 									data-placeholder="<?php echo $l->t('None set'); ?>">
-								<option value="all"><?php echo $l->t('All Users'); ?></option>
+								<option value="all" <?php if (isset($mount['applicable']['users']) && in_array('all', $mount['applicable']['users'])) echo 'selected="selected"';?> ><?php echo $l->t('All Users'); ?></option>
 								<optgroup label="<?php echo $l->t('Groups'); ?>">
 								<?php foreach ($_['groups'] as $group): ?>
 									<option value="<?php echo $group; ?>(group)"