A0-3232: Added automatic merge upstream job (#90)

This PR adds a job which performs fortnightly automatic job of upstream
merge. Here's example PR it produced:
#92

This PR is blocked by
#88

.github/workflows/automatic-upstream-merge-pr.yml

@@ -0,0 +1,31 @@
+---
+name: Automatic upstream merge
+
+on:
+  # run on 7th and 21th day of any month, ie fortnightly
+  schedule:
+    - cron: '1 0 7,21 * *'
+
+concurrency:
+  group: ${{ github.ref }}-${{ github.workflow }}
+  cancel-in-progress: false
+
+jobs:
+  main:
+    name: Merge polkadot-js/apps
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Checkout source code
+        uses: actions/checkout@v3
+
+      - name: Merge upstream repo
+        uses: Cardinal-Cryptography/github-actions/test-upstream-merge@v3
+        with:
+          target-repo: Cardinal-Cryptography/azero-dev
+          target-branch: alephzero
+          upstream-repo-url: https://github.com/polkadot-js/apps
+          upstream-branch: master
+          slack-webhook-url: ${{ secrets.SLACK_INCOMING_WEBHOOK_DEV_ONDUTY }}
+          git-commit-author: ${{ secrets.AUTOCOMMIT_AUTHOR }}
+          git-commit-email: ${{ secrets.AUTOCOMMIT_EMAIL }}
+          gh-ci-token: ${{ secrets.CI_GH_TOKEN }}