Send banner style as a string instead of an enum member

arkindex/templates/index.html

@@ -16,7 +16,7 @@
     {% endif %}
     {% if frontend_banner.message %}
     <meta name="banner_message" content="{{ frontend_banner.message }}">
-    <meta name="banner_style" content="{{ frontend_banner.style }}">
+    <meta name="banner_style" content="{{ frontend_banner.style.value }}">
     {% endif %}
     <title>ArkIndex {{ frontend_version }}</title>
     <link href="{{ cdn_assets_url }}/{{ frontend_version }}/arkindex-{{ frontend_version }}.css" rel="stylesheet">