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
For user-facing text, "Your locale" might be more in line with the other phrasing (e.g. "Your time zone," "Your site skin").