Issuable form due date

For some reason, this was never in the original merge request for due date?

![Screen_Shot_2016-04-22_at_21.49.44](/uploads/7d4e2b07f75a33009e3170fdb076a4bd/Screen_Shot_2016-04-22_at_21.49.44.png)

Closes #15516

See merge request !3895