Contact us page

@andrewgarner @aduggin 
It's not for the controller to decide what the breadcrumb trail should be. This is already taken care of by `https://github.com/moneyadviceservice/frontend/blob/master/app/domain/breadcrumb_trail.rb` - that object should continue to be the source of truth.

The problem to focus on is how we provide an override for the contact us, not how to override the breadcrumbs, canonical URL, alternative URLs etc.

I'd expect something like so:

```rb
def show

  # Fetch static page as normal
  @static_page = Core::StaticPageReader.new(params[:id]).call do
    not_found
  end

  # Fetch breadcrumbs as normal
  @breadcrumbs = BreadcrumbTrail.build(@static_page, category_tree)

  # Allow a template override
  if template_exists?("/static_pages/#{params[:id].underscore}")
    render template: "/static_pages/#{params[:id].underscore}"
  else
    render :show
  end
end
```