Downloads include WDPA T&Cs and Data Standard as required
PA Slugs include designation as well

This is because some PAs have the same name, varying only by
designation, which causes obvious issues for our slug URLs :)

Also moves the slug generation to a model method. I think it's better
placed here, and allows us access to all available PA attributes rather
than futzing around in the DataStandard class and sharing data.
Merge pull request #52 from unepwcmc/terms-and-conditions-for-downloads

Downloads include WDPA T&Cs and Data Standard as required