Service to comment whenever the approval of an assigned user is made

Approvals unlike comments saying +1 or LGTM do not generate any E-Mail or another notification method, which can cause the party that is waiting for approval to miss out when the approval is made.

Create a service that takes a *gitlab.MergeRequest event and checks for the Approved action with State open and send a nice message whenever any assigned person approves the merge request.

possible drawback includes having to assign someone before asking for approval