[milo] Add BuildSummary and common models.

milo/api/resp/builder.go

Index: milo/api/resp/builder.go
diff --git a/milo/api/resp/builder.go b/milo/api/resp/builder.go
index 46a30fe42a90870a7e15c6534760eff561072731..7b21dade48b2d513f35a5bbf797d520a9f9679b1 100644
--- a/milo/api/resp/builder.go
+++ b/milo/api/resp/builder.go
@@ -4,7 +4,11 @@
 
 package resp
 
-import "time"
+import (
+	"time"
+
+	"github.com/luci/luci-go/milo/appengine/common/model"
+)
 
 // Interval is a time interval which has a start, an end and a duration.
 type Interval struct {
@@ -21,7 +25,7 @@ type BuildSummary struct {
 	Link *Link
 
 	// Status of the build.
-	Status Status
+	Status model.Status
 
 	// Pending is time interval that this build was pending.
 	PendingTime Interval