RFR: 2395: Make it configurable to disable the contributor check for the /backport command

[Zhao Song]

In a discussion with Erik offline, he pointed out that the /backport command is different from all other commit commands. Instead of affecting the current repository, it only reads information from it and creates a branch in a fork of another repository. On GitHub, since anyone can comment on commits, it makes sense to have access control. However, in GitLab, where only authorized engineers can access the relevant repositories, we can safely disable the contributor check. In summary, it makes sense to make it configurable to disable the contributor check for the /backport command.

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Commit messages:
 - SKARA-2395

Changes: https://git.openjdk.org/skara/pull/1692/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1692&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2395
  Stats: 23 lines in 4 files changed: 19 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/skara/pull/1692.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1692/head:pull/1692

PR: https://git.openjdk.org/skara/pull/1692