Replace the "Alaveteli is Social Email" text on alaveteli.org

This replaces that text with "Alaveteli is an open source
Freedom of Information Platform".  There was much discussion
about what this text should be, but the eventually conclusion
was essentially that anything would be more accurate than
"Social Email" - not to say that isn't a great ambition for
a componentized version of Alaveteli, but it creates confusion
when we want to target the page at people who want to set up
FOI sites.

The new text looks like:

![screenshot](http://firefly.ukcod.org.uk/~mark/no-more-social-email.png)

This fixes mysociety/alaveteli#1019