CI: Add /rebase command (#1698)

(stolen from v3 repo)

Author: SaschaMann

.github/workflows/rebase.yml

@@ -0,0 +1,21 @@
+name: Rebase command
+
+on: 
+  issue_comment:
+    types: [created]
+
+jobs:
+  rebase:
+    name: Rebase
+    if: github.event.issue.pull_request != '' && contains(github.event.comment.body, '/rebase')
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@master
+      with:
+        fetch-depth: 0
+
+    - name: Automatic Rebase
+      uses: cirrus-actions/rebase@b91cdc98c0c0d3c4673647a74a68731bf3c29276 # 1.3.1
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}