URL-safe Base64 for use in routes and params

Two questions:
1) I used `Base64.urlsafe_encode64` instead because that seemed to make sense, that ok?
2) Should it locate with a base64 encoded signed gid? If so, I can add another commit for that.
Think a little clearer to test `assert_equal @person_sgid.to_s, @person_sgid.to_param`

We just care that `to_param` is the string representation.