Show a warning on the first page in case the user already configured web2

refs #7163

application/views/scripts/form/setup-welcome.phtml

@@ -1,6 +1,7 @@
 <?php
 
 use Icinga\Application\Icinga;
+use Icinga\Application\Config;
 use Icinga\Application\Platform;
 use Icinga\Web\Wizard;
 
@@ -33,10 +34,17 @@ $setupTokenPath = rtrim(Icinga::app()->getConfigDir(), '/') . '/setup.token';
       . t('insights', 'setup.welcome.screenshots.label') . '</a>'
     ); ?></p>
   </div>
+ <?php if (false === file_exists($setupTokenPath) && file_exists(Config::resolvePath('config.ini'))): ?>
+  <p class="restart-warning"><?= t(
+    'You\'ve already completed the installation of Icinga Web 2. Note that most of your configuration'
+      . ' files will be overwritten in case you\'ll re-configure Icinga Web 2 using this wizard!'
+  ); ?></p>
+ <?php else: ?>
   <p><?= t(
     'This wizard will guide you through the installation of Icinga Web 2. Once completed and successfully'
     . ' finished you are able to log in and to explore all the new and stunning features!'
   ); ?></p>
+ <?php endif ?>
   <form id="<?= $form->getName(); ?>" name="<?= $form->getName(); ?>" enctype="<?= $form->getEncType(); ?>" method="<?= $form->getMethod(); ?>" action="<?= $form->getAction(); ?>">
     <?= $form->getElement('token'); ?>
     <?= $form->getElement($form->getTokenElementName()); ?>

public/css/icinga/setup.less

@@ -306,6 +306,11 @@
     border: 1px solid lightgrey;
   }
 
+  p.restart-warning {
+    color: coral;
+    font-weight: bold;
+  }
+
   div.note {
     width: 40%;
     padding: 1em 1em 0;