Support specifying github actions in mergit.yml

[brandon-leapyear]

GitHub actions are represented in the GitHub API separately from the Commit statuses API. So we should support both.

# .mergit.yaml
expected:
  statuses:
    - ci/circleci: asdf
  workflows:
    "My workflow":
      - build
      - test

(transition: top-level statuses alias for expected: { statuses: ... })

Graphql query:

query {
  repository(...) {
    object(...) {
      ... on Commit {
        status {
          # "ci/circleci: asdf"
          contexts { context }
        }
        checkSuites(...) {
          nodes {
            workflowRun {
              # "My workflow"
              workflow { name }
            }
            checkRuns(...) {
              # "build" + status
              nodes { name, status }
            }
          }
        }
      }
    }
  }
}