RFR: 1824: /reviewers N should remove ready status for clean backports

[Zhao Song]

Currently, If a user issues /reviewers command in a clean backport, the reviewers requirement will not be displayed in the pr body and it won't be an integration blocker.

In this patch, after user issued /reviewers command, the reviewrs check for clean backport will be enabled. And if the Reviewers check requirements in the jcheck conf are stricter than the user's requirements, the bot will enforce the stricter requirements

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Commit messages:
 - fix a problem
 - SKARA-1824

Changes: https://git.openjdk.org/skara/pull/1482/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1482&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1824
  Stats: 108 lines in 3 files changed: 107 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1482.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1482/head:pull/1482

PR: https://git.openjdk.org/skara/pull/1482