Due date can be removed from milestones

## What does this MR do?

Adds a button to milestone form to allow user to remove due date 

## What are the relevant issue numbers?

Closes #15063 

## Screenshots (if relevant)

![Screen_Shot_2016-06-02_at_08.32.03](/uploads/8bd4446abde745dd5b1e140a887ec694/Screen_Shot_2016-06-02_at_08.32.03.png)

See merge request !4428