[Refactor] move APIFormat() of Issue and Label to convert package (#10423)

* Label: delete .APIFormat() and use convert.ToLabel()

* move issue APIFormat too

* add missing one

* move TEST too

* handle error -> return empty APIIssue

* Apply suggestions from code review

Co-Authored-By: guillep2k <18600385+guillep2k@users.noreply.github.com>

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: guillep2k <18600385+guillep2k@users.noreply.github.com>

modules/convert/pull.go

@@ -31,7 +31,7 @@ func ToAPIPullRequest(pr *models.PullRequest) *api.PullRequest {
 		return nil
 	}
 
-	apiIssue := pr.Issue.APIFormat()
+	apiIssue := ToAPIIssue(pr.Issue)
 	if pr.BaseRepo == nil {
 		pr.BaseRepo, err = models.GetRepositoryByID(pr.BaseRepoID)
 		if err != nil {