Total time spend in milestone
A total count of time spend in the milestone.
Display it in the right menu when on the milestone page.
Note that the units of the total time spent should be using the same automatic format as when you are entering / displaying time spent in the comment box of an issue, i.e. https://docs.gitlab.com/ce/workflow/time_tracking.html
Links / references
We need it to invoice clients based on the number of hours.
Make sure these are completed before closing the issue, with a link to the relevant commit.