Add webhooks for when a merge request is approved

• Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/12815

Requested feature

Right now we have webhooks for almost all events surrounding the merge request process, but we don;t have anything setup for when a merge request has been approved, thus this feature request.

Concrete questions / Next steps

I think there are two ways in which we could implement this:

• Fire a webhook every time someone approves
• Fire a webhook when the MR has reached the minimum number of approvers

I'm leaning for the second alternative, as it would create less noise and, in my opinion, it focuses on the actual event that is relevant for the status of the merge request.

@JobV what do you think of this?

/cc @vsizov