[Misc] Add tooling for backport automation

See also: https://forum.xwiki.org/t/automatic-backport-of-prs-on-branches/16075/

Author: surli

.github/workflows/backport.yml

@@ -0,0 +1,47 @@
+## ---------------------------------------------------------------------------
+## See the NOTICE file distributed with this work for additional
+## information regarding copyright ownership.
+##
+## This is free software; you can redistribute it and/or modify it
+## under the terms of the GNU Lesser General Public License as
+## published by the Free Software Foundation; either version 2.1 of
+## the License, or (at your option) any later version.
+##
+## This software is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+## Lesser General Public License for more details.
+##
+## You should have received a copy of the GNU Lesser General Public
+## License along with this software; if not, write to the Free
+## Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
+## 02110-1301 USA, or see the FSF site: http://www.fsf.org.
+## ---------------------------------------------------------------------------
+## Automatically backport merged pull requests that are labelled with a label containing "backport"
+## using https://github.com/tibdex/backport.
+name: Backport
+on:
+  pull_request_target:
+    types:
+      - closed
+      - labeled
+
+jobs:
+  backport:
+    name: Backport
+    runs-on: ubuntu-latest
+    # Only react to merged PRs for security reasons.
+    # See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target.
+    if: >
+      github.event.pull_request.merged
+      && (
+        github.event.action == 'closed'
+        || (
+          github.event.action == 'labeled'
+          && contains(github.event.label.name, 'backport')
+        )
+      )
+    steps:
+      - uses: tibdex/backport@v2
+        with:
+          github_token: ${{ secrets.GITHUB_TOKEN }}