Show the number of accounts in admin

See https://github.com/FreshRSS/FreshRSS/issues/679

app/views/user/manage.phtml

@@ -14,6 +14,12 @@
 			</div>
 		</div>
 
+		<div class="form-group">
+			<div class="group-controls">
+				<?php echo _t('admin.user.number', count(listUsers())); ?>
+			</div>
+		</div>
+
 		<div class="form-group form-actions">
 			<div class="group-controls">
 				<button type="submit" class="btn btn-important"><?php echo _t('gen.action.submit'); ?></button>