Enable PHPStan

- Fix error for level 0 & 1 (level 7 has 699 errors...)
- Add `updated_at` to site_credential (so the `timestamps()` method applies correctly)

tests/Wallabag/CoreBundle/Controller/RssControllerTest.php

@@ -11,7 +11,7 @@ class RssControllerTest extends WallabagCoreTestCase
         $doc = new \DOMDocument();
         $doc->loadXML($xml);
 
-        $xpath = new \DOMXpath($doc);
+        $xpath = new \DOMXPath($doc);
 
         if (null === $nb) {
             $this->assertGreaterThan(0, $xpath->query('//item')->length);