Add a basic React implementation of the map.

See client/README.md for instructions.

.github/workflows/lint.yml

@@ -0,0 +1,23 @@
+name: Build
+
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: windows-latest
+    steps:
+      - name: Check out Git repository
+        uses: actions/checkout@v2
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v1
+        with:
+          node-version: 12
+
+      - name: Install Node.js dependencies
+        run: npm ci
+
+      - name: Lint
+        run: |
+          cd client
+          npm run lint