Manual linking annotation not working

Hi

In https://github.com/heroku/heroku-buildpack-python/issues/1006 I added the annotation `@W-7828034@` to the issue description to try and manually link it to an existing work item.

However no comment was left of the issue from Git2Gus saying the work item has been linked. In addition no labels were added.

Looking at the implementation here:
https://github.com/forcedotcom/git2gus/blob/dbe8c53c7bf703d7607ca595ef0a473a843eb3b9/api/actions/linkToWorkItem.js#L68-L80

...it seems that Git2Gus tries to add labels before adding the comment, and that it uses the built-in label names and not the custom labels specified by `issueTypeLabels` (though I guess we'd need a reverse map due to one to many).

For this repository the default labels don't exist, so my thought was that the label addition was failing thus preventing Git2Gus from reaching the comment-adding stage.

However even after I manually created the `USER STORY` label (needed due to #88), and then removed and re-added the `@W-7828034@` annotation, Git2Gus still isn't linking the items.

I don't suppose there are any errors in the Git2Gus logs that can shed light on this? :-)