Serve robots.txt from the admin app

We’re getting rid of the PaaS proxy, which is where this file is
currently served from.

Values copied from 3632313fdf/ansible/roles/nginx/files/robots.txt

app/main/views/index.py

@@ -26,6 +26,17 @@ def index():
     return render_template('views/signedout.html')
 
 
+@main.route('/robots.txt')
+def robots():
+    return (
+        'User-agent: *\n'
+        'Disallow: /sign-in\n'
+        'Disallow: /support\n'
+        'Disallow: /support/\n'
+        'Disallow: /register\n'
+    ), 200, {'Content-Type': 'text/plain'}
+
+
 @main.route('/error/<int:status_code>')
 def error(status_code):
     if status_code >= 500: