RFR: 1797: Add support for /backport pull request command [v4]

[Zhao Song]

> This patch introduces support for the /backport command in pull requests. (ErikJ and ErikH proposed the idea and came up with this implementation.)
> 
> Usage of /backport in pull request:
> 
> Syntax: `/backport <repo> [<branch>] ` ` /backport disable <repo> [<branch>]` (default branch is master)
> 
> 1. If the `/backport` command is used in an **open** pull request, it adds a label `Backport=repo:branch` to the PR. If the PR is later integrated, the PR bot scans for backport labels and begins the backporting process. To cancel the backport, the user can use `/backport disable repo master` to remove the label before the PR is integrated.
> 2.  If the `/backport` command is used in an **integrated** PR, it creates the backport branch and comments on the PR. This process is similar to using the /backport command in commits. (This feature was implemented in [SKARA-1495](https://bugs.openjdk.org/browse/SKARA-1495) by guoxiong li, but the usage was not documented on the[ wiki page](https://wiki.openjdk.org/display/SKARA/Pull+Request+Commands), so seems like few people use this command)
> 3.  If the `/backport` command is used in a **closed** (not integrated) PR, the user will receive an error message stating that the command cannot be used in a closed but not integrated PR.

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  change backport label to lowercase

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

Changes:
  - all: https://git.openjdk.org/skara/pull/1466/files
  - new: https://git.openjdk.org/skara/pull/1466/files/26b9e7d9..65a75740

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1466&range=03
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1466&range=02-03

  Stats: 8 lines in 3 files changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/skara/pull/1466.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1466/head:pull/1466

PR: https://git.openjdk.org/skara/pull/1466