Create skeleton CampaignEdition model
Add large, medium and small image attachments for campaigns
Add fields for the campaign's organisation
Include the organisation details in fields_to_clone

Add the organisation data fields to the list of 
fields which are cloned when a new edition is
created.
Validate the brand colour and crest against a preset list

These keys maps directly to the CSS class names 
used by whitehall to style organisation logos.
Allow organisation_crest and organisation_brand_colour to be blank
Merge remote-tracking branch 'origin/add-campaigns'