UX: Hide link to incoming email when there is no bounce key

For example, webhooks mark email log entries as bounced without there ever being an incoming email.

app/serializers/email_log_serializer.rb

@@ -2,7 +2,8 @@ class EmailLogSerializer < ApplicationSerializer
   include EmailLogsMixin
 
   attributes :reply_key,
-             :bounced
+             :bounced,
+             :has_bounce_key
 
   has_one :user, serializer: BasicUserSerializer, embed: :objects
 
@@ -14,4 +15,8 @@ class EmailLogSerializer < ApplicationSerializer
   def reply_key
     @options[:reply_keys][[object.post_id, object.user_id]].delete("-")
   end
+
+  def has_bounce_key
+    object.bounce_key.present?
+  end
 end