Add migrate from OneDev (#16356)

* Use context to simplify logic.

* Added migration from OneDev.
This PR adds [OneDev](https://code.onedev.io/) as migration source.

Supported:
- [x] Milestones
- [x] Issues
- [x] Pull Requests
- [x] Comments
- [x] Reviews
- [x] Labels

modules/migrations/gogs_test.go

@@ -112,7 +112,7 @@ func TestGogsDownloadRepo(t *testing.T) {
 
 	// downloader.GetComments()
 	comments, _, err := downloader.GetComments(base.GetCommentOptions{
-		IssueNumber: 1,
+		Context: base.BasicIssueContext(1),
 	})
 	assert.NoError(t, err)
 	assertCommentsEqual(t, []*base.Comment{