Improve JIRA issue closing

Merged Drew Blessing requested to merge dblessing/gitlab-ee:improve_jira_issue_closing into master

Expected behavior

When a merge request has 'Fixes JIRA-1234' in the description the issue should not only be transitioned but also record a comment (regardless of workflow type in JIRA).

Observed behavior

Depending on the configured workflow a comment may or may not be allowed while transitioning to a new state. In my testing the newest JIRA default workflow did not allow comments while transitioning to 'Done', for example. The 'classic' workflow does, though.

This merge request splits the transition and comment in to two API calls. The advantage is that the comment will be recorded 100% of the time instead of it being dependent on the workflow configuration.

Also, JIRA returns a '204' response code when the transition is successful. I added '204' to the success conditional because we were generating an error in the current configuration even though the transition was successful.