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

javatests/google/registry/beam/InvoicingUtilsTest.java

@@ -49,7 +49,7 @@ public class InvoicingUtilsTest {
                 .withSuffix(".csv")
                 .withBaseFilename(
                     FileBasedSink.convertToFileResourceIfPossible(
-                        "my/directory/invoice_details_2017-10_registrar_tld")));
+                        "my/directory/2017-10/invoice_details_2017-10_registrar_tld")));
   }
 
   @Test