docs: Updated permissions. Tweaked install details

docs: Updated permissions. Tweaked install details

Added nginx to installation packages. We say that our install uses
nginx, so users should skip nginx if they want to use apache. Also,
our group permissions depend on a web server being installed.

Changed install directory for mediagoblin acct to /var/lib/mediagoblin
It works better if we need to back up data.

Assigned mediagoblin user to the same group as the web server. This
is needed so that the server can access uploaded media.

Created mediagoblin group for non-web-server things such as logs.