Refactor Slack Messages API

[caillou]

The Slack messages API is currently inconsistent.

// packages/slack/lib/publish.js
publish()
postMessage()

The publish() and postMessages() mehtods in https://github.com/orbiting/backends/blob/master/packages/slack/lib/publish.js do not return promises, nor do any of the methods in https://github.com/orbiting/backends/blob/master/packages/republik/lib/slack.js.

Yet, most of the code relies on these methods returning promises.

There is an attempt at refactoring the code here: 4ef98c2

Yet, the questions remains open, if these methods should return promises at all.

It is event questionable, if the Slack API should be used in our code, or if we should just log to the console and react to it separately.