Add organisation crests

Part of [this mainstream story](https://www.pivotaltracker.com/story/show/50363269) to provide a campaign page format in frontend. Organisation crests can be displayed in the campaign format, these are copies of [the Whitehall crests](https://github.com/alphagov/whitehall/tree/master/app/assets/images/crests) which should presumably be removed once this change is merged.