[milo] Add BuildSummary and common models.

milo/appengine/common/model/link.go

Index: milo/appengine/common/model/link.go
diff --git a/milo/appengine/common/model/link.go b/milo/appengine/common/model/link.go
new file mode 100644
index 0000000000000000000000000000000000000000..4d13aa7575a33bbfbf6de45c2d44e85645c17a9f
--- /dev/null
+++ b/milo/appengine/common/model/link.go
@@ -0,0 +1,14 @@
+// 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
+
+// Link denotes a single labeled link.
+type Link struct {
+	// Title (text) of the link.
+	Label string
+
+	// The destination for the link.
+	URL string
+}