Replace slugs with ids

Removes special handling of the `slug` field from collections in favour of using the `id` field. This means that urls in the UI are now of the form `/:collection/:id`.

Rather than trying to resolve conflicts with duplicate slugs we simply generate an id on behalf of the user. If a user still wants to have a `/persons/barack-obama` type slug url then they can create the entry via the API and specify a custom `id` for the record. However trying to create a record with a duplicate id will simply return an error rather than trying to resolve the conflict.

Needs https://github.com/mysociety/popit-api/pull/36 to be merged first in order to work correctly.

Fixes https://github.com/mysociety/popit/issues/175
Fixes https://github.com/mysociety/popit/issues/266
Fixes https://github.com/mysociety/popit-api/issues/15