Send a Mail to MR assignee when removing the WIP: status

Problem to solve

As an assignee of a Merge Request, I want to receive a mail when the WIP status of the MR is removed, so I know when I am able to merge the request.

Intended users

Developers, Code Reviewers

User experience goal

The assignee of a merge request should know once he is able to merge an MR.

Proposal

Once the WIP: status is removed from an MR, a mail should be sent to the assignee.