RFR: 351: /contributor command should allow GitHub and OpenJDK usernames

[Robin Westberg]

Hi all,

Please review this change that improves the usability of the /contributor command.

Best regards,
Robin

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Commit messages:
 - Improve usability of contributor command

Changes: https://git.openjdk.java.net/skara/pull/571/files
 Webrev: https://webrevs.openjdk.java.net/skara/571/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-351
  Stats: 131 lines in 3 files changed: 95 ins; 14 del; 22 mod
  Patch: https://git.openjdk.java.net/skara/pull/571.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/571/head:pull/571

PR: https://git.openjdk.java.net/skara/pull/571