RFR: 768: Skara bot should prevent attempt to use JBS backport issue ID in PR

[Robin Westberg]

Check that the issue type is of an allowed type.

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Commit messages:
 - Add warning regarding unknown issue types

Changes: https://git.openjdk.java.net/skara/pull/944/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=944&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-768
  Stats: 74 lines in 5 files changed: 6 ins; 61 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/944.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/944/head:pull/944

PR: https://git.openjdk.java.net/skara/pull/944