Add slug generator

after discussion with Alice, we agreed that clash resolution
is not needed as document titles must be unique as well.
Repository should generate slugs

using SlugGenerator
Remove slug generation from SpecialistDocument
Fix indentation
Move finder schema options out of specialist document

eventually these will come from the finder api
Give slug to SpecialistDocumentEdition

the slug is used by content api to find the editions for a particular artefact.
Merge pull request #22 from alphagov/refactor-slug-generation

Refactor slug generation