Add due_date field to issues
What does this MR do and why?
In gitlab-org/quality/triage-ops#1094 (closed), we would like to set an issue due_date
in order to decide whether we should close the issue or not.
The due_date
field is currently only accessible for milestones.
This MR exposes the due_date
field for issues, as well as an expired?
method for issues.