CorporateInformationPage of type AboutUs do not need a body

The "about us" information for organisations was extracted out of the `description` and `about_us` attributes on `Organisation` and turned into a new `CorporateInformationPage` type of `AboutUs`, setting the `summary` and `body` respectively. Because only `description` was required on `Organisation`, there are now a number of CIP AboutUs pages that do not have a `body` and are thus invalid. This change makes an `AboutUs` page without a body valid, which means editors will not be forced to supply a body for organisations that do not need one (e.g. closed or except organisations)

This also sets any `NULL` bodies to a blank string, which is what they would be set to if the editions were created from new using the admin interface.

https://www.pivotaltracker.com/story/show/71840958