ci: add rebase workflow

.github/workflows/rebase.yml

@@ -0,0 +1,20 @@
+name: Automatic Rebase
+on:
+  issue_comment:
+
+jobs:
+  rebase:
+    name: Rebase
+    if: github.event.issue.pull_request != '' && contains(github.event.comment.body, '@jellyfin rebase') && github.event.comment.author_association == 'MEMBER'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout the latest code
+        uses: actions/checkout@v2
+        with:
+          token: ${{ secrets.GH_TOKEN }}
+          fetch-depth: 0
+
+      - name: Automatic Rebase
+        uses: cirrus-actions/rebase@1.4
+        env:
+          GITHUB_TOKEN: ${{ secrets.GH_TOKEN }}