Remove logo from offsite org pages

Having logos for Organisations hosted off-site has a maintenance cost which we don't want.

This PR removes all logos from exempt organisations and adds a validation to prevent them being re-added.

Story: https://www.pivotaltracker.com/story/show/71117372

There is a ```exempt?``` method that does ```govuk_status == 'exempt'``` defined in this file that could be used here