Move invoice generation to billing bucket and improve emailing

This moves the new pipeline's invoice generation to the billing bucket, under the 'invoices/yyyy-MM' subdirectory.

This also changes the invoice e-mail to use a multipart message that attaches the invoice to the e-mail, to guarantee the correct MIME type and download.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181746191

java/google/registry/beam/InvoicingUtils.java

@@ -55,7 +55,8 @@ public class InvoicingUtils {
                     yearMonth ->
                         FileBasedSink.convertToFileResourceIfPossible(
                             String.format(
-                                "%s/%s", outputBucket, billingEvent.toFilename(yearMonth)))));
+                                "%s/%s/%s",
+                                outputBucket, yearMonth, billingEvent.toFilename(yearMonth)))));
   }
 
   /**