[API] extend StopWatch (#9196)

* squash api-stopwatch

* fix prepair logic! + add Tests

* fix lint

* more robust time compare

* delete responce 202 -> 204

* change http responce in test too

modules/structs/issue_stopwatch.go

@@ -0,0 +1,19 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package structs
+
+import (
+	"time"
+)
+
+// StopWatch represent a running stopwatch
+type StopWatch struct {
+	// swagger:strfmt date-time
+	Created    time.Time `json:"created"`
+	IssueIndex int64     `json:"issue_index"`
+}
+
+// StopWatches represent a list of stopwatches
+type StopWatches []StopWatch