Remove handling for legacy URLs.

Now that the URLs have been migrated, and redirects put in place, it's
no longer necessary to handle requests at the legacy URLs.
Merge pull request #9 from alphagov/cleanup_legacy_url

Remove handling for legacy URLs.
Set expiration time based on GDSApi::Response

www.agileplannerapp.com/boards/173808/cards/4800

Caching should be controllable from the origin of
the content. This will allow us to do things like
scheduled publishing better.