Create an opam-bot

[shonfeder]

So long as we have a GitHub based publication process, there some messages we will want to post directly on PRs. This topic has come up in several places, most recently in https://github.com/ocurrent/opam-repo-ci/issues/372

Even once we have need to mature beyond that basis, having a bot associated with opam.ci is likely to be useful. Moreover, we can use the opam-bot account to automate interactions with GitHub, which would help support adding a customized review/publication layer as an alternative (or augment) to the GitHub web UI (should that be desirable).