Add support to migrate from gogs (#14342)

Add support to migrate gogs:

  *  issues
  *  comments
  *  labels
  *  milestones
  *  wiki


Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Andrew Thornton <art27@cantab.net>

vendor/github.com/gogs/go-gogs-client/release.go

@@ -0,0 +1,22 @@
+// Copyright 2017 The Gogs 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 gogs
+
+import (
+	"time"
+)
+
+// Release represents a release API object.
+type Release struct {
+	ID              int64     `json:"id"`
+	TagName         string    `json:"tag_name"`
+	TargetCommitish string    `json:"target_commitish"`
+	Name            string    `json:"name"`
+	Body            string    `json:"body"`
+	Draft           bool      `json:"draft"`
+	Prerelease      bool      `json:"prerelease"`
+	Author          *User     `json:"author"`
+	Created         time.Time `json:"created_at"`
+}