Ask Google to not analyse the contents of the page for translation purposes (#681)

We already have i18n. Furthermore, Google may analyse sensitive content for
the purpose of recognising whether the page needs to be translated, see
https://support.google.com/webmasters/answer/79812?hl=en

index.html

@@ -24,6 +24,7 @@
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <meta name="referrer" content="no-referrer">
+    <meta name="google" content="notranslate">
 
     <title>Threema Web</title>
     <meta name="description" translate translate-attr-content="meta.DESCRIPTION"