Insert end date when closing a milestone

[AdmiringWorm]

Detailed Description

It would be useful for GRM to insert the end/due date of a milestone when closing the milestone issue when a date hasn't already been set.

Context

There are processes some follow to specify the end/due date when closing the milestone, having GRM do this automatically leaves one more thing not needed to be done manually.

Possible Implementation

I am not sure how it is implemented exactly, but it should AFAIK be possible on both GitHub and GitLab.

Your Environment

• Version Used: 0.16.0
• Edition Used (.NET Core, .NET Framework): .NET Framework
• Operating System and version (Windows 10, Ubuntu 18.04): Windows 10