Add support for multipart emails

Upgrade OutgoingEmail to permit subtypes to generate HTML message bodies
in addition to the existing text bodies. Where body types are provided,
they are combined using `multipart/alternative`.

Feature: Issue 3772
Change-Id: I64de701e35f22f499b068eff02e5154bca87943c

gerrit-server/src/main/resources/com/google/gerrit/server/mail/FooterHtml.soy

@@ -0,0 +1,20 @@
+/**
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+*/
+
+{namespace com.google.gerrit.server.mail.template}
+
+{template .FooterHtml autoescape="strict" kind="html"}
+{/template}