Remove logos from 'exempt' organisations

We don't want to have to maintain logos for exempt organisations, so we're
not going to show them.
Fix custom logo crest option label on organisation edit page

Organisations can be 'exempt' in two ways:
• Exemption from using the Single Identity branding
• Exemption from having to publish on GOV.UK

The label for the select option to use a Custom Logo was ambiguous as
to which of these exemptions it was referring to.
Validate that exempt organisations do not have custom logos
Merge pull request #1581 from alphagov/remove-logo-from-offsite-org-pages

Remove logo from offsite org pages