text: Title: Add SEO fields to product and category models and make editable in the dashboard
  Body: There are a few SEO components that we regularly find clients asking for the ability to control on product and category pages:
  
  1. Meta title
  2. Meta description
  3. Slug
  
  For products, slug, meta title and description are all auto-generated from the product title/description, with no option to override any of these. While this is fine a lot of the time, Oscar should provide the ability to override these.
  
  For categories, meta title and description are also auto-generated from the product title/description, and there is no option to override.
  
  I think we should have a "Search Engine Optimisation" tab on the dashboard edit views for both of these, which allow override of these parameters.