RFR: notify: rename SlackUpdater to SlackNotifier

[Erik Helin]

Hi all,

please review this refactoring that renames `SlackUpdater` to `SlackNotifier`.

Testing:
- [x] `make test` passes

Thanks,
Erik

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Commit messages:
 - notify: rename SlackUpdater to SlackNotifier

Changes: https://git.openjdk.java.net/skara/pull/649/files
 Webrev: https://webrevs.openjdk.java.net/skara/649/webrev.00
  Stats: 318 lines in 5 files changed: 158 ins; 159 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/649.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/649/head:pull/649

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