RFR: 1853: Make it possible to disable merge PRs for a repository

[Zhao Song]

In this patch, PR bot has been added with the capability to control whether merge-style pr is allowed in specific repositories.

By default, merge-style pr is enabled.

However, if it is desired to disable merge-style pr in a repo, the configuration can be added to the PR bot configuration as follows:

{
  "pr": {
      "repositories": {
          "repo1": {
               "merge": false
          }
       }
  }
}


If merge-style pr is disabled and the title of the pr matches the pattern of merge-style pr, the pr bot would reply a warning message to the user and prompt for the next steps.

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

Commit messages:
 - SKARA-1853

Changes: https://git.openjdk.org/skara/pull/1499/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1499&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1853
  Stats: 97 lines in 6 files changed: 95 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1499.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1499/head:pull/1499

PR: https://git.openjdk.org/skara/pull/1499