Add deploy previews

Using workflow from element-web

.github/workflows/build.yaml

@@ -1,5 +1,6 @@
 name: Build
 on:
+  pull_request: {}
   push:
     branches: [main]
 env:

.github/workflows/netlify-pr.yaml

@@ -0,0 +1,68 @@
+name: Netlify PR Preview
+on:
+    workflow_run:
+        workflows: ["Build"]
+        types:
+            - completed
+jobs:
+    deploy:
+        if: github.event.workflow_run.conclusion == 'success' && github.event.workflow_run.event == 'pull_request'
+        runs-on: ubuntu-latest
+        environment: Netlify
+        steps:
+            - name: 📝 Create Deployment
+              uses: bobheadxi/deployments@v1
+              id: deployment
+              with:
+                  step: start
+                  token: ${{ secrets.GITHUB_TOKEN }}
+                  env: Netlify
+                  ref: ${{ github.event.workflow_run.head_sha }}
+                  desc: |
+                      Do you trust the author of this PR? Maybe this build will steal your keys or give you malware.
+                      Exercise caution. Use test accounts.
+
+            - id: prdetails
+              uses: matrix-org/pr-details-action@v1.2
+              with:
+                  owner: ${{ github.event.workflow_run.head_repository.owner.login }}
+                  branch: ${{ github.event.workflow_run.head_branch }}
+
+            # There's a 'download artifact' action, but it hasn't been updated for the workflow_run action
+            # (https://github.com/actions/download-artifact/issues/60) so instead we get this mess:
+            - name: 📥 Download artifact
+              uses: dawidd6/action-download-artifact@v2
+              with:
+                  run_id: ${{ github.event.workflow_run.id }}
+                  name: previewbuild
+                  path: webapp
+
+            - name: ☁️ Deploy to Netlify
+              id: netlify
+              uses: nwtgck/actions-netlify@v1.2
+              with:
+                  publish-dir: webapp
+                  deploy-message: "Deploy from GitHub Actions"
+                  # These don't work because we're in workflow_run
+                  enable-pull-request-comment: false
+                  enable-commit-comment: false
+                  alias: pr${{ steps.prdetails.outputs.pr_id }}
+              env:
+                  NETLIFY_AUTH_TOKEN: ${{ secrets.NETLIFY_AUTH_TOKEN }}
+                  NETLIFY_SITE_ID: ${{ secrets.NETLIFY_SITE_ID }}
+              timeout-minutes: 1
+
+            - name: 🚦 Update deployment status
+              uses: bobheadxi/deployments@v1
+              if: always()
+              with:
+                  step: finish
+                  override: false
+                  token: ${{ secrets.GITHUB_TOKEN }}
+                  status: ${{ job.status }}
+                  env: ${{ steps.deployment.outputs.env }}
+                  deployment_id: ${{ steps.deployment.outputs.deployment_id }}
+                  env_url: ${{ steps.netlify.outputs.deploy-url }}
+                  desc: |
+                      Do you trust the author of this PR? Maybe this build will steal your keys or give you malware.
+                      Exercise caution. Use test accounts.