Fix deprecated hugo field names

I love churn

hugo.yaml

 baseURL: '/'
 title: Fachschaft Informatik
-paginate: 5
+pagination:
+  pagerSize: 5
 theme: hugo-arcana
 
 enableInlineShortcodes: true

layouts/partials/header.html

 <div id="header">
-    {{ if .Site.IsMultiLingual }}
+    {{ if hugo.IsMultilingual }}
         {{- partial "languages.html" . }}
     {{- end -}}
     <h1 id="logo_container">