Issue 4099 add a preference for the preferred locale for a user.

This adds the preferred locale user preference, currently the ability to edit this preference is controlled by roll out.

https://code.google.com/p/otwarchive/issues/detail?id=4099
Same comment as above regarding `then`, plus there's an extra space between the `if` and `Rails.env.test?`. It's probably a good idea to add dev, here, too, so developers can see it: `<% if Rails.env.test? || Rails.env.development? || $rollout.active?(:set_locale_preference, @user) %>`