Delete a service

- Puts a delete button at the bottom of every summary page
- Button takes supplier to same page with banner and button at top asking if they want to confirm deletion
- When deleted the user is sent back to dashboard

**Note: needs `grunt dev` running for styling of new banner to appear**
@TheDoubleK It's used here in the view: https://github.com/alphagov/supplier-submission-portal/blob/delete_a_service/app/views/Service/summaryPage.html#L60

The summary page doesn't have any way of knowing how many questions each page has. Instead it muddles its way through by looking for q1–q20 and seeing which ones have content. If it finds content then it can make a reasonable assumption that that question exists.