RFR: pr: update /backport command

[Erik Helin]

Hi all,

please review this patch that updates the `/backport` commit command. This new version will _not_ create the pull request, instead it provides a link that the user can click to create the pull request. That is, the bot will still cherry-pick the original commit into its own repo, but the the user will create the pull request with a branch in the bot's repository as the source branch. This has the large benefit that the PR author is the person doing the backport (in the old model the bot was the PR author, that came with a number of issues).

The bot will grant the backporter exclusive access to the branch with the backport in its repository in case the person doing the backport need to make additional changes.

Thanks,
Erik

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Commit messages:
 - pr: update /backport command

Changes: https://git.openjdk.java.net/skara/pull/1051/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1051&range=00
  Stats: 88 lines in 2 files changed: 30 ins; 25 del; 33 mod
  Patch: https://git.openjdk.java.net/skara/pull/1051.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1051/head:pull/1051

PR: https://git.openjdk.java.net/skara/pull/1051