Improve burndown chart algorithm

This updates the burndown chart to count issues as open when they are created in the middle of the milestone:

Before

All issues were to be opened at the beginning of the milestone(milestone.start_date) so the charts never goes up.

Now

Issues with created_at > milestone.start_date are considered open at the day they got created.
Every issue with created_at < milestone.start_date is considered open at the beginning of the milestone like before.

What does this MR do?

closes #6174 (closed)