Improve NGINX worker, connections, and body size

* Parameterize NGINX worker_processes to automatically reflect the
  number of CPU cores in the system.

* Set higher worker_connections, which should be fine given the
  number of allowed open file descriptors.

* Update some comments about client_max_body_size.  Explicitly
  declare this in the Wordpress NGINX virtual host, and make it
  consistent between blocks that relate to the frontend site.

I've deployed these configuration changes to staging, and all appears well.  I have tested an upload to the exhibitions site, to confirm that I haven't messed up the client maximum body size there.

8 MB sounds good.