Make sure urls in the API use record id not slug

To match the changes being made in popit we want to link to records
using their ids, not their slugs.
Remove slug deduplicating plugin from models

We no longer give slugs any special treatment, instead the user is
encouraged to set the `id` field via the API if they want a custom url.
Merge pull request #36 from mysociety/remove-slug-handling

Remove slug handling