Add migration from GitBucket (#16767)

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

Supported:
- Milestones
- Issues
- Pull Requests
- Comments
- Reviews
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There is no public usable instance so no integration tests added.

modules/structs/repo.go

@@ -242,13 +242,14 @@ type GitServiceType int
 
 // enumerate all GitServiceType
 const (
-	NotMigrated     GitServiceType = iota // 0 not migrated from external sites
-	PlainGitService                       // 1 plain git service
-	GithubService                         // 2 github.com
-	GiteaService                          // 3 gitea service
-	GitlabService                         // 4 gitlab service
-	GogsService                           // 5 gogs service
-	OneDevService                         // 6 onedev service
+	NotMigrated      GitServiceType = iota // 0 not migrated from external sites
+	PlainGitService                        // 1 plain git service
+	GithubService                          // 2 github.com
+	GiteaService                           // 3 gitea service
+	GitlabService                          // 4 gitlab service
+	GogsService                            // 5 gogs service
+	OneDevService                          // 6 onedev service
+	GitBucketService                       // 7 gitbucket service
 )
 
 // Name represents the service type's name
@@ -270,6 +271,8 @@ func (gt GitServiceType) Title() string {
 		return "Gogs"
 	case OneDevService:
 		return "OneDev"
+	case GitBucketService:
+		return "GitBucket"
 	case PlainGitService:
 		return "Git"
 	}
@@ -326,5 +329,6 @@ var (
 		GiteaService,
 		GogsService,
 		OneDevService,
+		GitBucketService,
 	}
 )