FR: Allow merges to be performed via personal access token regardless of push restriction

[f1sherman]

Unfortunately there is a gap in the Github/Jira integration that prevents it from closing tickets when a PR is merged by a bot/app:
atlassian/github-for-jira#403

Similar to how a personal access token can be provided to use for merging when there are push restrictions, it would be great if merging could happen via personal access token regardless of push restrictions, to work around this bug.

I understand if you don't want to allow this change given that the Jira functionality is arguably a bug. Since this would make bulldozer much more useful for us I thought I'd ask!