[milo] Add BuildSummary and common models.

milo/appengine/common/model/summary.go

Index: milo/appengine/common/model/summary.go
diff --git a/milo/appengine/common/model/summary.go b/milo/appengine/common/model/summary.go
new file mode 100644
index 0000000000000000000000000000000000000000..48e40f3f0da83d23c9d7294c85f92338d3645f40
--- /dev/null
+++ b/milo/appengine/common/model/summary.go
@@ -0,0 +1,23 @@
+// 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
+}