Allow merge request policies to have a block merge action

Release notes

Problem to solve

Customers should be able to completely block a merge request. Merge request approval policies require 1 or more approvals, and do not have a way to completely stop a merge request. Customers may have non-negotiable hard blockers that must be addressed.

Proposal

Add a new action type that blocks a merge. Some options for the name (non-prescriptive):

• block_merge_request
• require_changes
• deny

Intended users

TODO

Feature Usage Metrics

Does this feature require an audit event?