Handle the case where the current locale isn't in site list

We only really want to redirect people to live sites, so pre-launch
sites don't belong in WorldFOIWebsites. Handle the case where the
current locale isn't there. Closes #1345.
Merge branch 'feature/1345-other-country-fallback' into rails-3-develop
Merge branch 'rails-3-develop' of ssh://git.mysociety.org/data/git/public/alaveteli into rails-3-develop