Add translated worldwide offices.

This follows a very similar form to the translation for world locations, although
some of the admin interface is different.

Firstly, it should be noted that to avoid application downtime this commit doesn't
remove *any* columns from the worldwide_offices table. Once the application is
deployed and restarted, the vestigial content columns on that table should be
removed, but not before.
Translate labels on worldwide office page.
Don't hard-code 'en'
Now there *are* actually 3 columns.
Remove _fields partials, which weren't even being used.
Merge pull request #235 from alphagov/translate-worldwide-offices

Translate worldwide offices

https://www.pivotaltracker.com/story/show/44454707