Add support to migrate from gogs (#14342)

Add support to migrate gogs:

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

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

@@ -0,0 +1,24 @@
+// Copyright 2016 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 (
+	"bytes"
+	"encoding/json"
+	"fmt"
+)
+
+type AddOrgMembershipOption struct {
+	Role string `json:"role" binding:"Required"`
+}
+
+func (c *Client) AddOrgMembership(org, user string, opt AddOrgMembershipOption) error {
+	body, err := json.Marshal(&opt)
+	if err != nil {
+		return err
+	}
+	_, err = c.getResponse("PUT", fmt.Sprintf("/orgs/%s/membership/%s", org, user), jsonHeader, bytes.NewReader(body))
+	return err
+}