Adjust URL for creating variant product in dashboard

Why?
- s/_/-/
- I try and follow a RESTy approach to URLs where the parent identifier
  is to the left of any action or subordinates resources. (I'm sure
  there are other examples in Oscar where we don't follow this).
Move info block about editing a variant

I think the top of the page looks better than the left-hand navbar. It
avoids the mild spacing issues it had there too.
Improve headings/titles on product delete page

For variants, there was a missing breadcrumb and the wording made it
sound like you were deleting the parent.
Fix bug with handling last child deletions

This change:

- Alters the logical flow so the deletion of products takes place in the
  `delete` method rather than `get_success_url`. As per SRP, I don't
  think the `get_success_url` method should write to the DB.

- Fixes a logical error around child product deletions. We need to test
  if the product being deleted is a last child *before* deleting it as
  this changes the result.