allow setting a base-path for the web UI and API (#583)

frontend/index.html

@@ -9,6 +9,7 @@
     <script>
       // global config, will be overridden by backend if available
       let WGPORTAL_BACKEND_BASE_URL="http://localhost:5000/api/v0";
+      let WGPORTAL_BASE_PATH="";
       let WGPORTAL_VERSION="unknown";
       let WGPORTAL_SITE_TITLE="WireGuard Portal";
       let WGPORTAL_SITE_COMPANY_NAME="WireGuard Portal";