Sealed Sender support

https://signal.org/blog/sealed-sender/

js/modules/metadata/CiphertextMessage.js

@@ -0,0 +1,13 @@
+module.exports = {
+  CURRENT_VERSION: 3,
+
+  // This matches Envelope.Type.CIPHERTEXT
+  WHISPER_TYPE: 1,
+  // This matches Envelope.Type.PREKEY_BUNDLE
+  PREKEY_TYPE: 3,
+
+  SENDERKEY_TYPE: 4,
+  SENDERKEY_DISTRIBUTION_TYPE: 5,
+
+  ENCRYPTED_MESSAGE_OVERHEAD: 53,
+};