Refactor common email sending utility

The main thrust of this is to create a common POJO that contains email content in a simple way, then have one class that converts that to an email and sends it. Any class that uses email should only have to deal with creating that POJO.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=237883643

java/google/registry/util/EmailMessage.java

@@ -0,0 +1,94 @@
+// Copyright 2019 The Nomulus Authors. All Rights Reserved.
+//
+// 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.
+
+package google.registry.util;
+
+import com.google.auto.value.AutoValue;
+import com.google.common.collect.ImmutableList;
+import com.google.common.net.MediaType;
+import java.util.Collection;
+import java.util.Optional;
+import javax.mail.internet.InternetAddress;
+
+/** Value class representing the content and metadata of an email. */
+@AutoValue
+public abstract class EmailMessage {
+
+  public abstract String subject();
+  public abstract String body();
+  public abstract ImmutableList<InternetAddress> recipients();
+  public abstract InternetAddress from();
+  public abstract Optional<InternetAddress> bcc();
+  public abstract Optional<MediaType> contentType();
+  public abstract Optional<Attachment> attachment();
+
+  public static Builder newBuilder() {
+    return new AutoValue_EmailMessage.Builder();
+  }
+
+  public static EmailMessage create(
+      String subject, String body, InternetAddress recipient, InternetAddress from) {
+    return newBuilder()
+        .setSubject(subject)
+        .setBody(body)
+        .setRecipients(ImmutableList.of(recipient))
+        .setFrom(from)
+        .build();
+  }
+
+  /** Builder for {@link EmailMessage}. */
+  @AutoValue.Builder
+  public abstract static class Builder {
+
+    public abstract Builder setSubject(String subject);
+    public abstract Builder setBody(String body);
+    public abstract Builder setRecipients(Collection<InternetAddress> recipients);
+    public abstract Builder setFrom(InternetAddress from);
+    public abstract Builder setBcc(InternetAddress bcc);
+    public abstract Builder setContentType(MediaType contentType);
+    public abstract Builder setAttachment(Attachment attachment);
+
+    abstract ImmutableList.Builder<InternetAddress> recipientsBuilder();
+
+    public Builder addRecipient(InternetAddress value) {
+      recipientsBuilder().add(value);
+      return this;
+    }
+
+    public abstract EmailMessage build();
+  }
+
+  /** An attachment to the email, if one exists. */
+  @AutoValue
+  public abstract static class Attachment {
+
+    public abstract MediaType contentType();
+    public abstract String filename();
+    public abstract String content();
+
+    public static Builder newBuilder() {
+      return new AutoValue_EmailMessage_Attachment.Builder();
+    }
+
+    /** Builder for {@link Attachment}. */
+    @AutoValue.Builder
+    public abstract static class Builder {
+
+      public abstract Builder setContentType(MediaType contentType);
+      public abstract Builder setFilename(String filename);
+      public abstract Builder setContent(String content);
+      public abstract Attachment build();
+    }
+  }
+}