RFR: 1858: Make it possible to configure to require reviewers for "Merge" PRs

[Zhao Song]

In this patch, PR bot has been added with the capability to control the requirement of reviews for merge pull requests.

By default, review is not necessary for merge pull requests.

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

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


Currently, jcheck would run in two places in skara bot. One is in checkRun and one is in IntegrateCommand. The basic idea for us to ignore reviewersCheck for CleanBackport and Merge PullRequest is ignoring the error message.

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Commit messages:
 - Merge branch 'master' into SKARA-1858
 - SKARA-1858

Changes: https://git.openjdk.org/skara/pull/1495/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1495&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1858
  Stats: 155 lines in 11 files changed: 136 ins; 5 del; 14 mod
  Patch: https://git.openjdk.org/skara/pull/1495.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1495/head:pull/1495

PR: https://git.openjdk.org/skara/pull/1495