add CONTACT_MAILBOX contact type

bramble-api/src/main/java/org/briarproject/bramble/api/contact/ContactType.java

@@ -23,7 +23,12 @@ public enum ContactType {
 	/**
 	 * The owner of a mailbox.
 	 */
-	MAILBOX_OWNER(2);
+	MAILBOX_OWNER(2),
+
+	/**
+	 * A contact's mailbox
+	 */
+	CONTACT_MAILBOX(3);
 
 	private final int value;