Trigger auto-merge through the Github API

[joao-paulo-parity]

Github implemented auto-merging this year and according to https://github.community/t/enabling-auto-merge-on-a-pr-using-the-api/150797/5 it is possible to enable it through the GraphQL API.

Instead of having processbot tell if the statuses are passing through Webhook Events, which is e.g. susceptible to webhook delivery failures, it would be more reliable to enable/disable auto-merge through the GraphQL API.

[joao-paulo-parity]

Related: https://github.com/EmbarkStudios/octobors#why

[joao-paulo-parity]

I don't think this would work as we're hoping for because some jobs spontaneously fail, as per #374 (comment), and that would cancel the automerge. Then, to re-enable the automerge in those scenarios we'd still rely on webhook deliveries.

I don't know if using automerge would provide a better flow than what we have currently, but I guess it's relevant to keep this issue open until a decision has been made about this.