MM-18060: Include deleted posts in compliance export. (#12957)

model/message_export.go

@@ -21,6 +21,7 @@ type MessageExport struct {
 	PostId         *string
 	PostCreateAt   *int64
 	PostUpdateAt   *int64
+	PostDeleteAt   *int64
 	PostMessage    *string
 	PostType       *string
 	PostRootId     *string