Import topics during migration (#7851)

* add GetTopics interface

* CreateTopics

* remove un-needed comment

docs/content/doc/advanced/migrations.en-us.md

@@ -34,6 +34,7 @@ create a Downloader.
 ```Go
 type Downloader interface {
 	GetRepoInfo() (*Repository, error)
+	GetTopics() ([]string, error)
 	GetMilestones() ([]*Milestone, error)
 	GetReleases() ([]*Release, error)
 	GetLabels() ([]*Label, error)