feat: allow runtime configuration (#654)

public/index.html

@@ -4,7 +4,6 @@
       <title>Authn | <%= process.env.SITE_NAME %></title>
       <meta charset="utf-8">
       <meta name="viewport" content="width=device-width, initial-scale=1.0">
-      <link rel="shortcut icon" href="<%=htmlWebpackPlugin.options.FAVICON_URL%>" type="image/x-icon" />
       <% if (process.env.OPTIMIZELY_URL) { %>
         <script
           src="<%= process.env.OPTIMIZELY_URL %>"