NginX: Restrict access to sensitive directories

[xurizaemon]

Discussed in #21

• NginX does not respect .htaccess
• Solution (until we move sensitive directories from webroot) is to use nginx config to prevent access
• Directory paths are specific to CMS (eg sites//files/civicrm/ versus wp-content/something)
• But since the risk of collision is low and AMP is targeted at local use, IMO it's OK to block the lot.