Added deploy ghost theme github action

- see https://github.com/marketplace/actions/deploy-ghost-theme
- use the new deploy theme github action to automatically deploy master to a test site

.github/workflows/deploy-theme.yml

@@ -0,0 +1,11 @@
+name: Deploy Theme
+on: [push]
+jobs:
+  deploy:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@master
+      - uses: TryGhost/action-deploy-theme@v1.0.0
+        with:
+          api-url: ${{ secrets.GHOST_ADMIN_API_URL }}
+          api-key: ${{ secrets.GHOST_ADMIN_API_KEY }}