[infra] Integrate with external PR preview system

.github/workflows/detect_pull_request_preview.yml

@@ -0,0 +1,24 @@
+name: pr-preview-detect
+on: deployment
+jobs:
+  detect-deployment:
+    runs-on: ubuntu-18.04
+    steps:
+    - uses: actions/checkout@v1
+      # By default, the "checkout" Action will attempt to check out the
+      # revision to be deployed. Because it does not fetch GitHub Pull Request
+      # branches, this will fail.
+      with:
+        ref: refs/heads/master
+    - name: Install dependency
+      run: pip install requests
+    - name: Detect deployment
+      run:
+        ./tools/ci/pr_preview.py
+          --host https://api.github.com
+          --github-project web-platform-tests/wpt
+          detect
+          --target https://wptpr.live
+          --timeout 600
+      env:
+        GITHUB_TOKEN: ${{ secrets.DEPLOY_TOKEN }}

.github/workflows/pull_request_previews.yml

@@ -0,0 +1,27 @@
+name: pr-preview-sync
+on:
+  schedule:
+    - cron: */5 * * * *
+jobs:
+  update-pr-preview:
+    runs-on: ubuntu-18.04
+    steps:
+    - uses: actions/checkout@v1
+    - name: Install dependency
+      run: pip install requests
+    - name: Synchronize state
+      run:
+        ./tools/ci/pr_preview.py
+          --host https://api.github.com
+          --github-project web-platform-tests/wpt
+          synchronize
+          --window 480
+      env:
+        # This Workflow must trigger further workflows. The GitHub-provided
+        # `GITHUB_TOKEN` secret is incapable of doing this [1], so a
+        # user-generated token must be specified instead. This token requires
+        # the "repo" scope, and is should be stored as a Secret named
+        # "DEPLOY_TOKEN" in this GitHub project.
+        #
+        # [1] https://help.github.com/en/github/automating-your-workflow-with-github-actions/events-that-trigger-workflows
+        GITHUB_TOKEN: ${{ secrets.DEPLOY_TOKEN }}