Idea: Use the Status API

[DilumAluthge]

The pull request build needs to save state, because the cron build will later retrieve that state. The state consists of:

1. Did this PR meet all of the automerge guidelines? Yes or No.
2. If it met all of the automerge guidelines, what was the SHA of the PR's head branch at the time of approval?

Currently, we use pull request reviews as a way of saving this state. But I think that it would be more robust to use either the Statuses API or the Checks API in order to save and retrieve this state.

The GitHub API reference says:

The Checks API functionality is available exclusively to your GitHub Apps.

So I think we should use the Statuses API instead.

[DilumAluthge]

I am eternally grateful to the authors of GitHub.jl: https://github.com/JuliaWeb/GitHub.jl/blob/master/src/repositories/statuses.jl