RFR: notify: rename SlackUpdater to SlackNotifier

[Robin Westberg]

On Wed, 10 Jun 2020 09:59:14 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this refactoring that renames `SlackUpdater` to `SlackNotifier`.
> 
> Testing:
> - [x] `make test` passes
> 
> Thanks,
> Erik

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/649