[Do Not Merge] Full manual install guide

WIP on a full manual install guide.

I've stripped out most of the explanation text to make the guide as concise as possible for testing. Most commands are now non-interactive, but some still require interaction.

Some notable differences to the current guide:

- Specifically create alaveteli user
- Set permissions on files
- Use `/opt` as the install prefix

We need to decide what we recommend as the production app and web server setup. My long-term preference is Unicorn with nginx, because:

- The unicorn config is contained in the application
- Its common to use sysvinit with unicorn so the application can be run as a service, which makes managing it easier
- The app and web server are decoupled, which makes it easier to change and scale; nginx only has to proxy to a port on some host.
- In my experience, nginx has considerably better performance
- Passenger / nginx requires using a custom-compiled version of nginx, although it seems a Debian repository is available for this.