Less constrained merge method access

[xmo-odoo]

Apparently some teams have decided nobody should have self r+, which then becomes an issue with merge methods as those are tied to r+ rights, and reviewers setting merge methods is ???

• missing merge method notifications is delayed (part to the linked PR status check which is currently hourly)
• merge method requirement is unreliable due to github's inconsistent behaviour around some events (and issue_comment doesn't provide it as far as I can tell)
• an author can not set the merge method on their PRs if they don't have self-r+ rights

Possibilities:

• put the merge method check on a shorter cron and batch query github to validate that the PR is not squash
• add a new ACL / capability (based on what?) to allow "trusted" authors to set the merge method, maybe other commands would be moved to that (up from author or down from reviewer)
• just allow authors to set the merge method, this does allow some amount of unreviewed changes (set PR message and set merge method to merge or squash) but there is no deeply functional impact...