RFR: 1727: Ability to control whether clean backports require review

[Zhao Song]

In this patch, PR bot has been added with the capability to control the requirement of reviews for clean backports. 

By default, no review is necessary for clean backports. 

However, if it is desired to enable the requirement of reviews for backports in a repo, the configuration can be added to the PR bot configuration as follows:

{
  "pr": {
      "repositories": {
          "repo1": {
               "requiresReviewForBackport": true
          }
       }
  }
}

-------------

Commit messages:
 - fix a problem
 - fix comment
 - SKARA-1727

Changes: https://git.openjdk.org/skara/pull/1457/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1457&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1727
  Stats: 141 lines in 7 files changed: 126 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/skara/pull/1457.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1457/head:pull/1457

PR: https://git.openjdk.org/skara/pull/1457