Add CI job to label PRs with merge conflicts (mastodon#22020)

.github/workflows/rebase-needed.yml

@@ -0,0 +1,17 @@
+name: PR Needs Rebase
+
+on:
+  push:
+  pull_request_target:
+    types: [synchronize]
+
+jobs:
+  label-rebase-needed:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check for merge conflicts
+        uses: eps1lon/actions-label-merge-conflict@releases/2.x
+        with:
+          dirtyLabel: 'rebase needed :construction:'
+          repoToken: '${{ secrets.GITHUB_TOKEN }}'
+          commentOnDirty: This pull request has merge conflicts that must be resolved before it can be merged.