Add "Needs Triage" label to new PRs

@singingwolfboy god I hope this is the last thing I've overlooked.
Where is `issue_url` defined? And are you sure this is the URL for the Github issue, rather than the JIRA issue? Also, the API endpoint docs say that the `labels` attribute should be a list of strings, not a list of JSON objects: https://developer.github.com/v3/issues/#edit-an-issue