Trigger merge request webhook when `detailed_merge_status` mergeable
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## Problem to Solve
Asynchronous checks for `detailed_merge_status` mean that the API needs to be continually polled to see if a merge request is mergeable. This creates problems in automation it generates excessive requests and bad information that needs to be checked before merging can happen.
## Proposal
When the `detailed_merge_status` check is complete, a webhook should be triggered to inform listening services of it's updated status.
issue