[CLIENT-2160] CI/CD: add workflow to fast forward master to stage branch (#374)

Author: juliannguyen4

.github/workflows/ff-master-stage.yml

@@ -0,0 +1,23 @@
+on:
+  issue_comment:
+    types: [created]
+
+jobs:
+  fast_forward_job:
+    name: Fast Forward
+    if: github.event.issue.pull_request != '' && contains(github.event.comment.body, '/fast-forward')
+    runs-on: ubuntu-latest
+    steps:
+      # To use this repository's private action, you must check out the repository
+      - name: Checkout
+        uses: actions/checkout@v2
+      # Basic use case example
+      - name: Fast Forward PR
+        id: ff-action
+        uses:  ./
+        with:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          success_message: 'Success! Fast forwarded master to stage! ```git checkout master && git merge stage --ff-only``` '
+          failure_message: 'Failed! Cannot do fast forward!'
+          staging_branch: 'stage'
+          production_branch: 'master'