Check Redmine ticket is closed when using "refs"

[domcleal]

When a PR is opened that uses "refs" to refer to a ticket number, suggest that "fixes" be used when the ticket is open. refs is usually used to add commits to an existing, closed ticket.

Ideally it'd also check that when refs is used against a closed ticket that the ticket hasn't already shipped, and if it has, to request that the dev opens a new ticket and uses "fixes".