Use a GitHub action to build the Netlify preview build

So we don't need netflify to sit there building the develop branch
for no reason. Also it will comment with the URL rather than having
to dig it out of the checks.

Upload part not tested yet.

.github/workflows/layered-build.yaml

@@ -0,0 +1,19 @@
+name: Layered Preview Build
+on:
+    pull_request:
+        branches: [develop]
+jobs:
+    build:
+        runs-on: ubuntu-latest
+        steps:
+            - uses: actions/checkout@v2
+            - name: Build
+              run: scripts/ci/layered.sh && cd element-web && cp element.io/develop/config.json config.json && CI_PACKAGE=true yarn build
+            - name: Upload Artifact
+              uses: actions/upload-artifact@v2
+              with:
+                  name: Preview Build
+                  path: element-web/webapp
+                  # We'll only use this in a triggered job, then we're done with it
+                  retention-days: 1
+

.github/workflows/netflify.yaml

@@ -0,0 +1,32 @@
+name: Upload Preview Build to Netlify
+on:
+    workflow_run:
+        workflows: ["Layered Preview Build"]
+        types:
+            - completed
+jobs:
+    build:
+        runs-on: ubuntu-latest
+        if: >
+            ${{ github.event.workflow_run.event == 'pull_request' &&
+            github.event.workflow_run.conclusion == 'success' }}
+        steps:
+            - name: Download Artifact
+              uses: actions/download-artifact@v2
+              with:
+                  name: "Preview Build"
+            - name: Deploy to Netlify
+              uses: nwtgck/actions-netlify@v1.2
+              with:
+                  publish-dir: .
+                  github-token: ${{ secrets.GITHUB_TOKEN }}
+                  alias: deploy-preview-${{ github.event.number }}
+                  deploy-message: "Deploy from GitHub Actions"
+                  enable-pull-request-comment: true
+                  enable-commit-comment: false
+                  overwrites-pull-request-comment: true
+              env:
+                  NETLIFY_AUTH_TOKEN: ${{ secrets.NETLIFY_AUTH_TOKEN }}
+                  NETLIFY_SITE_ID: ${{ secrets.NETLIFY_SITE_ID }}
+              timeout-minutes: 1
+