[milo] Add BuildSummary and common models.

milo/appengine/common/model/summary.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
+
+import "time"
+
+// Summary summarizes a thing (step, build, group of builds, whatever).
+type Summary struct {
+        // Status indicates the 'goodness' and lifetime of the thing. This usually
+        // translates directly to a status color.
+        Status Status
+
+        // Start indicates when this thing started doing its action.
+        Start time.Time
+
+        // End indicates when this thing completed doing its action.
+        End time.Time
+
+        // Text is a possibly-multi-line summary of what happened.
+        Text []string
+}