RFR: 1852: Make it possible to disable backport PRs for repository

[Zhao Song]

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

By default, backport is enabled.

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

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


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

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

Commit messages:
 - fix a problem
 - SKARA-1852

Changes: https://git.openjdk.org/skara/pull/1497/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1497&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1852
  Stats: 101 lines in 6 files changed: 97 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1497.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1497/head:pull/1497

PR: https://git.openjdk.org/skara/pull/1497