[milo] Add BuildSummary and common models.

milo/appengine/common/model/patch_info.go

+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package model
+
+// PatchInfo provides information about a patch included in a build.
+type PatchInfo struct {
+        // A link to the patch page (i.e. the gerrit/rietveld review page).
+        Link Link
+
+        // The email of the author of the Patch.
+        AuthorEmail string
+}