Preserve locale when moving around the site.

By introspecting which controllers can handle locales, we can target
our patch around named routes to only add locales for routes generated
to those controllers.

I've changed the cucumber features for world locations and offices
to exercise changing translations via the interface, in order to
drive inclusion of the locale links on those pages. I've also made
the step usage a bit more consistent, using the language name to drive
clicking on links to translations rather than jumping straight to the
locale.

https://www.pivotaltracker.com/story/show/44452983