Refactor about.php

- Hardcode source code URL and credits in about template, remove from DB/admin interface; only contacts remains editable
- Merge apropos editables into one À propos section, remove charte, add editable source code URL

app/public/admin/actions/page.php

@@ -13,7 +13,7 @@ if (!isset($_POST['csrf_token'], $_SESSION['csrf_token'])
     exit;
 }
 
-$allowedSlugs = ['about', 'licenses', 'charte'];
+$allowedSlugs = ['about', 'licenses'];
 $slug         = $_POST['slug'] ?? '';
 $content      = $_POST['content'] ?? '';