Add: [Actions] preview workflow, where PRs can be reviewed in the browser

When a developer attaches the "preview" label, a build is created
and published on https://preview.openttd.org/. After that, new
pushes to the PR are automatically build (as long as the "preview"
label exists).

If a non-developer attaches the "preview" label, it will be
removed.

.github/workflows/preview_label.yml

@@ -0,0 +1,66 @@
+name: Preview label
+
+on:
+  pull_request_target:
+    types:
+    - labeled
+
+env:
+  TEAM_CORE_DEVELOPER: core-developers
+
+jobs:
+  check_preview_label:
+    name: Check for preview label
+    if: github.event.action == 'labeled' && github.event.label.name == 'preview'
+
+    runs-on: ubuntu-20.04
+
+    steps:
+    - name: Check if label was added by core developer
+      id: core_developer
+      continue-on-error: true
+      uses: octokit/request-action@v2.x
+      with:
+        route: GET /orgs/OpenTTD/teams/${{ env.TEAM_CORE_DEVELOPER }}/memberships/${{ github.event.sender.login }}
+      env:
+        GITHUB_TOKEN: ${{ secrets.PREVIEW_GITHUB_TOKEN }}
+
+    - if: steps.core_developer.outcome == 'failure'
+      name: Remove preview label if not core developer
+      uses: octokit/request-action@v2.x
+      with:
+        route: DELETE /repos/{owner}/{repo}/issues/{issue_number}/labels/preview
+        owner: ${{ github.event.repository.owner.login }}
+        repo: ${{ github.event.repository.name }}
+        issue_number: ${{ github.event.number }}
+      env:
+        GITHUB_TOKEN: ${{ secrets.PREVIEW_GITHUB_TOKEN }}
+
+    - if: steps.core_developer.outcome == 'success'
+      name: Create deployment
+      id: deployment
+      uses: octokit/request-action@v2.x
+      with:
+        route: POST /repos/{owner}/{repo}/deployments
+        mediaType: |
+          previews:
+          - ant-man
+          - flash
+        owner: ${{ github.event.repository.owner.login }}
+        repo: ${{ github.event.repository.name }}
+        ref: ${{ github.event.pull_request.head.sha }}
+        task: deploy:preview
+        auto_merge: false
+        required_contexts: "[]"
+        environment: preview-pr-${{ github.event.number }}
+        description: "Preview for Pull Request #${{ github.event.number }}"
+      env:
+        GITHUB_TOKEN: ${{ secrets.PREVIEW_GITHUB_TOKEN }}
+
+    - if: steps.core_developer.outcome == 'success'
+      name: Trigger 'preview build'
+      uses: peter-evans/repository-dispatch@v1
+      with:
+        token: ${{ secrets.PREVIEW_GITHUB_TOKEN }}
+        event-type: "Preview build #${{ github.event.number }}"
+        client-payload: '{"folder": "pr${{ github.event.number }}", "sha": "${{ github.event.pull_request.head.sha }}", "deployment_id": "${{ fromJson(steps.deployment.outputs.data).id }}"}'