Set up continuous integration using GitHub Actions

This also uses the unofficial Bash strict mode for shell scripts.

.github/workflows/ci.yml

@@ -0,0 +1,52 @@
+name: Continuous integration
+on:
+  - push
+  - pull_request
+
+jobs:
+  test:
+    name: Test
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2.3.4
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '7.3'
+          tools: composer:v1
+          extensions: curl, fileinfo, gd, mbstring, openssl, pdo, pdo_mysql, xml, zip
+          # Disable Xdebug to improve performance.
+          coverage: none
+
+      - name: Install October and set up MySQL database
+        run: |
+          sudo systemctl start mysql
+          echo -e '[client]\nuser="root"\npassword="root"' > "$HOME/.my.cnf"
+          mysql -e 'CREATE DATABASE IF NOT EXISTS `database`;'
+          # https://stackoverflow.com/questions/52364415/php-with-mysql-8-0-error-the-server-requested-authentication-method-unknown-to
+          mysql -e "ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY 'root';"
+
+          mkdir -p october
+          pushd october
+          php -r "eval('?>'.file_get_contents('https://octobercms.com/api/installer'));"
+          php artisan key:generate
+          # Set up the database.
+          sed -i "s/'password'   => ''/'password'   => 'root'/g" config/database.php
+          sed -i "s/'activeTheme' => 'demo'/'activeTheme' => 'godotengine'/g" config/cms.php
+          php artisan october:up
+
+          cp -r ../plugins/* plugins
+          cp -r ../themes/* themes
+          php artisan plugin:refresh paulvonzimmerman.patreon
+          env -C plugins/paulvonzimmerman/patreon composer install
+          php artisan plugin:refresh pikanji.agent
+          php artisan plugin:refresh rainlab.blog
+          php artisan plugin:refresh serenitynow.cacheroute
+          php artisan plugin:refresh sobored.rss
+
+      - name: Run Lighthouse CI
+        run: |
+          npm install @lhci/cli@0.6.x
+          npx lhci autorun