Improve CMS and PostgreSQL restarts

- Add playbook for restarting postgresql_dbs server.  This puts the
  frontend and API into maintenance mode.
- Update the siteproxy Nginx configuration to provide custom 503
  error responses whenever the CMS server behind it is rebooted.

Further notes:
- When restarting PHP (as opposed to rebooting the CMS server), the
  reload should happen without disruption.
- The whole frontend site has to be taken offline even though only the
  personalization and login features use the database, because Rails
  will throw a database connection exception.
- I'm also lowering Nginx's proxy_connect_timeout to say that we expect
  to be able to form a tcp/ip connection to the application server
  within 5 seconds.  We'll present an error page if we can't do that.
  This should be OK for our setup.