🚚 Send message to Discord when deploying to hedy.org (#5841)

Send a message to Discord once we deploy to production. 

Fixes #5742

**How to test**

* We have to accept and deploy, and then will see if this does send the message.

.github/workflows/deploy-to-prod.yml

@@ -17,3 +17,11 @@ jobs:
           heroku_api_key: ${{secrets.HEROKU_API_KEY}}
           heroku_app_name: hedy-beta
           heroku_email: [email protected]
+      - name: "Send message on Discord"
+        if: success()
+        uses: discord-actions/message@v2
+        with:
+          webhookUrl: ${{ secrets.DISCORD_WEBHOOK }}
+          username: "Github"
+          avatar: "https://github.githubassets.com/images/modules/logos_page/GitHub-Mark.png"
+          message: "Deployed to hedy.org"