add luacheck and fix common errors

also fixes https://github.com/pandorabox-io/pandorabox.io/issues/493

.github/workflows/luacheck.yml

@@ -0,0 +1,18 @@
+name: luacheck
+
+on: [push, pull_request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: apt
+      run: sudo apt-get install -y luarocks
+    - name: luacheck install
+      run: luarocks install --local luacheck
+    - name: luacheck run
+      run: $HOME/.luarocks/bin/luacheck ./