RFR: notify: rename IssueUpdater to IssueNotifier

[Erik Helin]

Hi all,

please review this refactoring that renames `IssueUpdater` to `IssueNotifier`.

Testing:
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - notify: rename IssueUpdater to IssueNotifier

Changes: https://git.openjdk.java.net/skara/pull/650/files
 Webrev: https://webrevs.openjdk.java.net/skara/650/webrev.00
  Stats: 1194 lines in 10 files changed: 597 ins; 596 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/650.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/650/head:pull/650

PR: https://git.openjdk.java.net/skara/pull/650