Autodetect rpi, improve nginx config

A few improvements to the RaspberryPi experience:

1. During install, the optimizerpi script is optionally triggered (if CONFIG_PACKAGE is RPi)
2. No-longer necessary to specify CONFIG_PACKAGE - this is now autodetected by OS=Linux, Hardware = arm6l and CENTRAL_SERVER =False.  This behavior can be over-ridden by specifying CONFIG_PACKAGE="" in local_settings.
3. NGINX port settings are now detected from PRODUCTION_PORT and a new setting PROXY_PORT.  Unfortunately,  nginx cannot reference environment variables, so if these settings are changed, it is necessary to re-run optimizerpi.  PROXY_PORT is only used if config=RPi and the optimizerpi script has been run.
4. NGINX configuration now exists in sites-available, with a link in sites-enabled  (this is the more standard way of doing things
5. NGINX 502 error is normally caused by the webserver not running.  A basic, custom 502 error has been implemented to improve the student experience with this error.
6. Install script - adjustments for word-wrapping on narrower terminals