RFR: 1655: A PR/commit with an invalid jcheck config should be caught by jcheck

[Zhao Song]

SKARA-1655 describes a problem related with invalid jcheck config in a new change.

Currently, Skara never checks if the jcheck config is changed in a new change. So for example, if we accidentally
changed the jcheck config in a change and it was bad formatted, the bot will be in a retry loop and stuck.

In this patch, `JCheckConfCheck` is added to JCheck. If this check is configured, jcheck will check whether the jcheck config is still valid in the new change. If it is not, the specific problem will be added to the PR body and this PR will be blocked to be `ready for review`.

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Commit messages:
 - SKARA-1655

Changes: https://git.openjdk.org/skara/pull/1415/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1415&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1655
  Stats: 363 lines in 9 files changed: 362 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1415.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1415/head:pull/1415

PR: https://git.openjdk.org/skara/pull/1415