Fixes #8181

This commit includes the configuration file for an action that:

* When a PR that has a label like "Backport <branch>" is merged,
  it will trigger a backport action
* If the PR commits merge cleanly into the target branch, a new PR will
  be created against it, assigned to the same user as the merged one.
  The usual checks will be run on the new PR
* If the commits don't merge cleanly, a comment will be posted on the
  orginal PR with the manual commands to fix the conflicts, and the PR
  will be labelled with "Backport failed"
* Additionally, Tech Team members can trigger a backport on open or
  already closed PRs adding a comment starting with `/backport` (and
  adding the relevant label)

Some more details:
* The action is run using a token from the @ckanbot account, this needs
  to be stored in the repository secrets (`BACKPORT_ACTION_PAT`)
* There are also two public variables (`TECH_TEAM_USER_IDS` and
  `CKANBOT_USER_ID`) that need to be added to the repository variables.
* The action used source code code can be found at
  https://github.com/korthout/backport-action