nginx configuration for long site names

Three fixes in here needed for Carnegie / CME

1. Add support for specifying a "template_subdir" var that will be used when setting up the nginx sites-available.  If it's absent then no problem, but if it's there, look in that subdirectory of the templates dir first.  

2. Install our own nginx.conf.  This way we can specify a wider site_name_hash line, required with really long site names (like ```preview.class.claslite.carnegiescience.edu```)

3. Noticed that perms were wrong on sites-available files, fixed those while I was in there.

@jbau pls review