Set due date for individual issues

Report http://feedback.gitlab.com/forums/176466-deprecated-feedback-forum/suggestions/6239210-set-due-dates-for-individual-issues

and @AshleyS really want this to move from Asana

I don't think that requiring that people make a milestone is a good alternative, it takes a lot of time to create one.

/cc @dzaporozhets @JobV