Create static robots.txt file, remove dynamic route

Fixes https://gitlab.com/timvisee/send/-/issues/17

app/routes.js

@@ -9,7 +9,6 @@ module.exports = function(app = choo({ hash: true })) {
   app.route('/unsupported/:reason', body(require('./ui/unsupported')));
   app.route('/error', body(require('./ui/error')));
   app.route('/blank', body(require('./ui/blank')));
-  app.route('/robots.txt', function() {return 'User-agent: * Disallow: /'});
   app.route('/oauth', function(state, emit) {
     emit('authenticate', state.query.code, state.query.state);
   });

public/robots.txt

@@ -0,0 +1,2 @@
+User-agent: *
+Disallow: /