RFR: 1814: Add logic to allow bot run Backport Command

[Zhao Song]

In [SKARA-1797](https://bugs.openjdk.org/browse/SKARA-1797), the bot might issue /backport command when the pr is integrated. 

However, in the very first of Backport Command, the bot would check whether the command user is in the census. The bot itself is not in the census, so the command issued by the bot would fail.

In this patch, I added the logic to explicitly allow bot run `/backport` command in closed pr or commits.

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Commit messages:
 - SKARA-1814

Changes: https://git.openjdk.org/skara/pull/1468/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1468&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1814
  Stats: 3 lines in 3 files changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1468.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1468/head:pull/1468

PR: https://git.openjdk.org/skara/pull/1468