Get rid of ALAVETELI_USER and ALAVETELI_HOME

It's not clear whether they are actual variables (they aren't).
Fix Postfix configuration instructions.

Copy the file changes used by install_site.sh
Fix a couple of typos
Bit more info on what you should see on the happy path.
Reference Postfix as well as Exim - it's easier to setup.
Configure postfix non-interactively

And add the hostname to the domains to be delivered locally.
Don't focus on Ubuntu

Where Ubuntu and Debian differ, give both details.
Highlight path and user name
Link config vars.
Use default path
Add note about restarting postfix
Give Debian filename too
Use the example domain, not localhost
Merge branch 'gh-pages-postfix' of ssh://git.mysociety.org/data/git/public/alaveteli into gh-pages-postfix