Support for handling legacy PP protected area URLs

The Protected Areas URLs in the previous version were in the format
`/sites/<slug>`. This commit introduces the same route, and a table that
matches the old legacy slugs with WDPA IDs.
Documentation for legacy PP handling
Add missing structure dump
Legacy PA urls match all PA sub-pages

Such as species
Merge pull request #60 from unepwcmc/legacy-urls

Legacy URLs