add GitHub workflow for validate_static

Uses a GitHub-hosted runner in order to run validate_static for all pull
requests and pushes to master. See
https://help.github.com/en/actions/getting-started-with-github-actions/about-github-actions
for more details.

.github/workflows/validate-static.yml

@@ -0,0 +1,47 @@
+name: Validate static
+
+on:
+  pull_request:
+  push:
+    branches: [master]
+
+jobs:
+  validate-static:
+    runs-on: ubuntu-latest
+    
+    env:
+      VALIDATOR_VERSION: 20.3.16
+    
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions/setup-node@v1
+    
+    - name: Cache node modules
+      uses: actions/cache@v1
+      env:
+        cache-name: cache-node-modules
+      with:
+        path: ~/.npm 
+        key: ${{ runner.os }}-build-${{ env.cache-name }}-${{ hashFiles('**/package-lock.json') }}
+
+    - name: Cache validator
+      id: validator-cache
+      uses: actions/cache@v1
+      with:
+        path: vnu-runtime-image
+        key: ${{ runner.os }}-validator-${{ env.VALIDATOR_VERSION }}
+
+    - run: sudo apt-get -y install libxml2-utils
+    - run: npm install
+
+    - name: Download validatornu
+      # There isn't a package with a `validatornu` command in Ubuntu, so download the validator
+      # from the GitHub releases and change the validate_static script later to use it.
+      run: wget https://github.com/validator/validator/releases/download/${{ env.VALIDATOR_VERSION }}/vnu.linux.zip && unzip vnu.linux.zip
+      if: steps.validator-cache.outputs.cache-hit != 'true'
+
+    - run: sed -i 's+validatornu+vnu-runtime-image/bin/vnu+g' validate_static
+
+    - name: validate static
+      run: ./validate_static
+