⏪️(helm) bring back helm chart

This is a revert of 1da5a removing actual deployments and keeping
only the dev environment in Tilt.

The clean-up was a bit heavy handed. We should keep the Helm
chart to the development repository and move away only the
deployment configuration.

.github/workflows/helmfile-linter.yaml

@@ -0,0 +1,22 @@
+name: Helmfile lint
+run-name: Helmfile lint
+
+on:
+  pull_request:
+    branches:
+      - 'main'
+
+jobs:
+  helmfile-lint:
+    runs-on: ubuntu-latest
+    container:
+      image: ghcr.io/helmfile/helmfile:latest
+    steps:
+      -
+        uses: numerique-gouv/action-helmfile-lint@main
+        with:
+          app-id: ${{ secrets.APP_ID }}
+          age-key: ${{ secrets.SOPS_PRIVATE }}
+          private-key: ${{ secrets.PRIVATE_KEY }}
+          helmfile-src: "src/helm"
+          repositories: "impress,secrets"