Add workflow for backporting PRs (#4912)

* Add workflow for backporting PRs

- Time saving for backporting bug fixes to stable branches
- Apply to PRs before closing

* Cleanup .json file

backportrc.json

@@ -0,0 +1,8 @@
+{
+  "repoOwner": "Oliver Walters",
+  "repoName": "InvenTree",
+  "targetBranchChoices": [],
+  "branchLabelMapping": {
+    "^backport-to-(.+)$": "$1"
+  }
+}