[milo] Add BuildSummary and common models.

milo/appengine/common/model/status_string.go

+// Code generated by "stringer -type=Status"; DO NOT EDIT.
+
+package model
+
+import "fmt"
+
+const _Status_name = "NotRunRunningSuccessFailureWarningInfraFailureExceptionExpiredDependencyFailureWaitingDependency"
+
+var _Status_index = [...]uint8{0, 6, 13, 20, 27, 34, 46, 55, 62, 79, 96}
+
+func (i Status) String() string {
+        if i < 0 || i >= Status(len(_Status_index)-1) {
+                return fmt.Sprintf("Status(%d)", i)
+        }
+        return _Status_name[_Status_index[i]:_Status_index[i+1]]
+}