RFR: 2367: Customize bot messages based on forge

[Zhao Song]

When an invalid user attempts to issue a Skara command, the Skara bot should print a customized message that is based on the forge.

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Commit messages:
 - SKARA-2367

Changes: https://git.openjdk.org/skara/pull/1685/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1685&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2367
  Stats: 30 lines in 4 files changed: 11 ins; 12 del; 7 mod
  Patch: https://git.openjdk.org/skara/pull/1685.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1685/head:pull/1685

PR: https://git.openjdk.org/skara/pull/1685