Fix improper HTMLURL usages in Go code (#22839)

In Go code, HTMLURL should be only used for external systems, like
API/webhook/mail/notification, etc.

If a URL is used by `Redirect` or rendered in a template, it should be a
relative URL (aka `Link()` in Gitea)

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/web/repo/repo.go

@@ -344,7 +344,7 @@ func acceptOrRejectRepoTransfer(ctx *context.Context, accept bool) error {
 		ctx.Flash.Success(ctx.Tr("repo.settings.transfer.rejected"))
 	}
 
-	ctx.Redirect(ctx.Repo.Repository.HTMLURL())
+	ctx.Redirect(ctx.Repo.Repository.Link())
 	return nil
 }