Add conditional rb for when to show the skip-to-main-navigation link

@andrewgarner 
I would also leave it up controllers inheriting from `ApplicationController` to overwrite the helper method. At the moment `ApplicationController` knows a bit too much about classes that inherit it.

```rb
class ApplicationController < ActionController::Base
  def display_skip_to_main_navigation?
    true
  end
end

class HomeController < ApplicationController
  def display_skip_to_main_navigation?
    false
  end

  helper_method :display_skip_to_main_navigation?
end
```