Add migration from GitBucket (#16767)

This PR adds [GitBucket](https://gitbucket.github.io/) as migration source.

Supported:
- Milestones
- Issues
- Pull Requests
- Comments
- Reviews
- Labels

There is no public usable instance so no integration tests added.

modules/migrations/gitbucket.go

@@ -0,0 +1,72 @@
+// Copyright 2021 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 migrations
+
+import (
+	"context"
+	"net/url"
+	"strings"
+
+	"code.gitea.io/gitea/modules/migrations/base"
+	"code.gitea.io/gitea/modules/structs"
+)
+
+var (
+	_ base.Downloader        = &GitBucketDownloader{}
+	_ base.DownloaderFactory = &GitBucketDownloaderFactory{}
+)
+
+func init() {
+	RegisterDownloaderFactory(&GitBucketDownloaderFactory{})
+}
+
+// GitBucketDownloaderFactory defines a GitBucket downloader factory
+type GitBucketDownloaderFactory struct {
+}
+
+// New returns a Downloader related to this factory according MigrateOptions
+func (f *GitBucketDownloaderFactory) New(ctx context.Context, opts base.MigrateOptions) (base.Downloader, error) {
+	u, err := url.Parse(opts.CloneAddr)
+	if err != nil {
+		return nil, err
+	}
+
+	baseURL := u.Scheme + "://" + u.Host
+	fields := strings.Split(u.Path, "/")
+	oldOwner := fields[1]
+	oldName := strings.TrimSuffix(fields[2], ".git")
+
+	return NewGitBucketDownloader(ctx, baseURL, opts.AuthUsername, opts.AuthPassword, opts.AuthToken, oldOwner, oldName), nil
+}
+
+// GitServiceType returns the type of git service
+func (f *GitBucketDownloaderFactory) GitServiceType() structs.GitServiceType {
+	return structs.GitBucketService
+}
+
+// GitBucketDownloader implements a Downloader interface to get repository information
+// from GitBucket via GithubDownloader
+type GitBucketDownloader struct {
+	*GithubDownloaderV3
+}
+
+// NewGitBucketDownloader creates a GitBucket downloader
+func NewGitBucketDownloader(ctx context.Context, baseURL, userName, password, token, repoOwner, repoName string) *GitBucketDownloader {
+	githubDownloader := NewGithubDownloaderV3(ctx, baseURL, userName, password, token, repoOwner, repoName)
+	githubDownloader.SkipReactions = true
+	return &GitBucketDownloader{
+		githubDownloader,
+	}
+}
+
+// SupportGetRepoComments return true if it supports get repo comments
+func (g *GitBucketDownloader) SupportGetRepoComments() bool {
+	return false
+}
+
+// GetReviews is not supported
+func (g *GitBucketDownloader) GetReviews(context base.IssueContext) ([]*base.Review, error) {
+	return nil, &base.ErrNotSupported{Entity: "Reviews"}
+}