Support GitHub's new multiple assignment feature

[Fryguy]

https://github.com/blog/2178-multiple-assignees-on-issues-and-pull-requests?utm_source=twitter&utm_medium=social&utm_campaign=multiple-assignees

Also verify that the current assignment isn't broken.

[miq-bot]

This issue has been automatically marked as stale because it has not been updated for at least 3 months.

If you can still reproduce this issue on the current release or on master, please reply with all of the information you have about it in order to keep the issue open.

Thank you for all your contributions! More information about the ManageIQ triage process can be found in the traige process documentation.