Uses FileUtils for import clean up

`#rm_rf` allows us to delete directories, as well as files (.gdb is a
dir)
Release handler also removes the wdpa source import table
Renames S3Downloader to S3

This is to reflect that it is less of a tool, and more a representation
of some arbitrary WDPA storage medium with download operations.
Removes unnecessary requirement of creating S3 instance before using it
Remove secrets.yml and add an example placeholder with instructions
Refactors Ogr::Postgres to not require manual instance creation

Laziness and beauty!