Validate the presence of the description.

It's required in [PublicBody#type_of_authority]( https://github.com/mysociety/alaveteli/blob/rails-3-develop/app/models/public_body.rb#L343-L347)
Merge branch '2000-validate-category-description' into rails-3-develop
Merge branch 'rails-3-develop' into release/0.20