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/navigation.py

@@ -194,6 +194,7 @@ class HeaderNavigation(Navigation):
         'resend_email_verification',
         'resume_service',
         'revoke_api_key',
+        'robots',
         'send_messages',
         'send_notification',
         'send_one_off',
@@ -451,6 +452,7 @@ class MainNavigation(Navigation):
         'resend_email_verification',
         'resume_service',
         'roadmap',
+        'robots',
         'security',
         'send_notification',
         'service_dashboard_updates',
@@ -638,6 +640,7 @@ class CaseworkNavigation(Navigation):
         'resume_service',
         'revoke_api_key',
         'roadmap',
+        'robots',
         'security',
         'send_messages',
         'send_notification',
@@ -859,6 +862,7 @@ class OrgNavigation(Navigation):
         'resume_service',
         'revoke_api_key',
         'roadmap',
+        'robots',
         'security',
         'send_messages',
         'send_notification',