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/user_follow.go

@@ -0,0 +1,47 @@
+// Copyright 2015 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 "fmt"
+
+func (c *Client) ListMyFollowers(page int) ([]*User, error) {
+	users := make([]*User, 0, 10)
+	return users, c.getParsedResponse("GET", fmt.Sprintf("/user/followers?page=%d", page), nil, nil, &users)
+}
+
+func (c *Client) ListFollowers(user string, page int) ([]*User, error) {
+	users := make([]*User, 0, 10)
+	return users, c.getParsedResponse("GET", fmt.Sprintf("/users/%s/followers?page=%d", user, page), nil, nil, &users)
+}
+
+func (c *Client) ListMyFollowing(page int) ([]*User, error) {
+	users := make([]*User, 0, 10)
+	return users, c.getParsedResponse("GET", fmt.Sprintf("/user/following?page=%d", page), nil, nil, &users)
+}
+
+func (c *Client) ListFollowing(user string, page int) ([]*User, error) {
+	users := make([]*User, 0, 10)
+	return users, c.getParsedResponse("GET", fmt.Sprintf("/users/%s/following?page=%d", user, page), nil, nil, &users)
+}
+
+func (c *Client) IsFollowing(target string) bool {
+	_, err := c.getResponse("GET", fmt.Sprintf("/user/following/%s", target), nil, nil)
+	return err == nil
+}
+
+func (c *Client) IsUserFollowing(user, target string) bool {
+	_, err := c.getResponse("GET", fmt.Sprintf("/users/%s/following/%s", user, target), nil, nil)
+	return err == nil
+}
+
+func (c *Client) Follow(target string) error {
+	_, err := c.getResponse("PUT", fmt.Sprintf("/user/following/%s", target), jsonHeader, nil)
+	return err
+}
+
+func (c *Client) Unfollow(target string) error {
+	_, err := c.getResponse("DELETE", fmt.Sprintf("/user/following/%s", target), nil, nil)
+	return err
+}