Fix for en locales in urls

We noticed 2 things:

1. featuring an edition that can't be translated (e.g. a policy) would result in urls with ?locale=en in them.
2. clicking through results in a 500 error

So we

a) don't generate locale=en links when they're not needed
b) don't 500 if they are there

No pivotal for this.

ps.  Do review the commits individually, there are some meaty notes in one of the messages.