RFR: 2508: IssueNotifier should handle inactive JBS users

[Zhao Song]

This patch is trying to let IssueNotifier skip setting the assignee to an inactive user, otherwise, the bot will be stuck.

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Commit messages:
 - SKARA-2508

Changes: https://git.openjdk.org/skara/pull/1720/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1720&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2508
  Stats: 21 lines in 1 file changed: 12 ins; 7 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1720.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1720/head:pull/1720

PR: https://git.openjdk.org/skara/pull/1720