Rename slug_url property to url

Since we're removing slugs from the urls it makes sense to rename this
method in the models.
Use url method in the views
Remove slug handling from models

If a user wants to add a slug field to their models manually, they still
can, but we no longer do any special handling of that field. Also remove
the deduplicate_slug function.
Route to apps using the model id instead of slug

Since the slug is not a unique field in the database and it doesn't
appear in the popolo schema we're replacing it with the id field. Users
can still specify a custom id if they'd like to have "pretty" urls, but
when they do that using the id field the uniqueness is enforced.
Change suggestions list to use id

Sadly we can't use the model's 'url' function in this view because it's
also rendered clientside by Backbone, which doesn't (yet) have that
method on the models.
Update fixtures to use custom ids

This means that we're testing the custom id functionality by default and
also means there's no need to change all the url assertions in the
tests.
Don't ask for a slug when creating records

When creating people or organizations don't ask the user to enter a
slug, instead leave mongo to generate an id for the record. If custom
ids are required then the user has to create records using the API.
Fix grammar in organization template

I've also updated the comment in the template as it took me a moment to
figure out what a 'bb view' was.
Remove inconsistent image indexing test

This test fails more often than it passes due to elasticsearch's refresh
taking longer than the next request does.

In the future this can potentially be added back in as a faster test
using javascript with mocha/supertest when we have access to
elasticsearch so we can force a refresh.
Merge pull request #341 from mysociety/replace-slug-with-id

Replace slugs with ids