Improved keyword sanitizer

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -54,6 +54,7 @@ import java.net.IDN;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 import java.security.NoSuchAlgorithmException;
+import java.text.Normalizer;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
@@ -2063,7 +2064,9 @@ public class MessageHelper {
                 continue;
             sb.append(kar);
         }
-        return sb.toString();
+
+        return Normalizer.normalize(sb.toString(), Normalizer.Form.NFKD)
+                .replaceAll("[^\\p{ASCII}]", "");
     }
 
     static String sanitizeEmail(String email) {