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 #54 from unepwcmc/slug-with-designation

PA Slugs include designation as well