Add "approved" event for MRs

Summary

Event model [doesn't support approved event] which can happen for MRs, so we can't track easily when an MR was approved. It's required as part of #7168 (closed) so we should add it. Please see riginal discussion #7168 (comment 327105946)

Proposal

Introduce an "approved" Event type, such that a record is created every time a Merge Request is approved by someone.

Improvements

New event type. New opportunities for analytics.

Risks

no risks

Involved components

app/models/event.rb