Allow per-app configuration of robots.txt

https://developers.google.com/search/reference/robots_txt has
the specification

Part of #344

setup/start/nginx/appconfig.ejs

@@ -82,6 +82,12 @@ server {
         # Disable check to allow unlimited body sizes
         client_max_body_size 0;
 
+<% if (robotsTxt) { %>
+        location = /robots.txt {
+            return 200 "<%= robotsTxt %>";
+        }
+<% } %>
+
 <% if ( endpoint === 'admin' ) { %>
         location /api/ {
             proxy_pass   http://127.0.0.1:3000;