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
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.
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 on 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.