RFR: SKARA-758: Add /check pull request command

[Kartik Ohri]

SKARA-758: Add /check pull request command

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Commit messages:
 - Add /check command to manually execute jcheck

Changes: https://git.openjdk.java.net/skara/pull/954/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=954&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-758
  Stats: 24 lines in 2 files changed: 23 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/954.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/954/head:pull/954

PR: https://git.openjdk.java.net/skara/pull/954