Add Merge Request Approval Subscription

[shakeandbakebaby]

When a merge request gets approved it is an indicator to go do something else. This request is for adding the ability to subscribe to merge request is approved or unapproved. Current work around is to have a webhook post the change and not the gitlab mattermost plugin.

[MatthewDorner]

I would like to work on this.

[MatthewDorner]

The merge request approval and unapproval are still MergeEvent type webhook events, so there would be no additional subscription feature. You'd just add the code in webhook/merge_request.go to handle the approved and unapproved actions and generate the correct DM and Channel posts. That sound right?