Changing list subscriber endpoints

[supun-io]

Currently, list_ids are taken in in subscription create and update endpoints. This is limited. This update will change it to have the following endpoints:

• POST /subscribers/{id}/lists
  • list name or id
  • if_unsubscribed (what to do if the user has previously unsubscribed from the list: error | force_create)
• DELETE /subscribers/{id}/lists
  • list name or id
  • reason:
    • unsubscribe - unsubscribed by the user. A record will be added to a table so that we can later check for if_unsubscribed above
    • other - other reason, no special behavior (generally when deleting from the Console)