RFR: 1858: Make it possible to configure to require reviewers for "Merge" PRs [v2]

[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 jcheck configuration as follows:
> 
> [checks "reviewers"]
> reviewers=1
> merge=check
> 
> 
> Currently, jcheck would run in two places in skara bot. One is in checkRun and one is in IntegrateCommand. For the merge pull requests that not require review, the Reviewers check would be skipped.
> 
> The other way to run jcheck in using SKARA CLI. This patch is also compatible with it.

Zhao Song has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  SKARA-1858

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Changes:
  - all: https://git.openjdk.org/skara/pull/1495/files
  - new: https://git.openjdk.org/skara/pull/1495/files/2293892a..02c54dfb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1495&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1495&range=00-01

  Stats: 262 lines in 12 files changed: 105 ins; 139 del; 18 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