Issue due in current month when updating

Fixes incorrect due date in issue description text when updating.

When updating a retrospective issue at the end of the milestone we want the due date to be the 26th of the current month.

When creating a retrospective issue before the milestone starts we want the due date to be the 26th of the next month (no change).

See also:

1. https://about.gitlab.com/handbook/engineering/workflow/#product-development-timeline
2. https://gitlab.com/gitlab-org/async-retrospectives/pipeline_schedules

Closes #34 (closed)