[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' of the thing. This usually translates

[Ryan Tseng, 2017/06/17 00:44:28]

On top of "goodness" It also describes where in the lifetime it is (pending,
running, etc)

[iannucci, 2017/06/17 01:30:04]

Done.

+        // directly to a status color.
+        Status Status
+
+        // Start indicates when this thing started doing its action.
+        Start time.Time

[Ryan Tseng, 2017/06/17 00:44:28]

Make this a pointer, nil to indicate still pending / not yet started

[iannucci, 2017/06/17 01:30:04]

Won't work in datastore. 0 value of time.Time is pretty unique though.

+
+        // End indicates when this thing completed doing its action.
+        End time.Time

[Ryan Tseng, 2017/06/17 00:44:28]

Make this a pointer, to indicate no end time / still running.

+
+        // Text is a possibly-multi-line summary of what happened.
+        Text []string
+}