RFR: notify: add Slack notifier

[Robin Westberg]

On Mon, 6 Apr 2020 08:28:39 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> this is a draft pull request to add Slack support to the `NotifyBot`. @rwestberg
> can you help out with implementing the `NotifyBot` part of
> `handleNewPullRequest`?
> 
> Thanks,
> Erik

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/564