feat: new client-friendly subscribers endpoint

[supun-io]

closes #434
closes #319
closes #447

Changes

• subscriber_list_removals records when a subscriber is removed from a list
• Single POST /subscribers endpoint to create or update a subscriber
• PATCH /subscribers/:id removed
• Subscriber UNSUBSCRIBED status removed (columns are not yet removed, only the entities)
  • subscribers.unsubscribed_at removed
  • subscribers.opt_in_at no longer used - but still saved when confirming subscription
• New GET /subscribers/email/{email} endpoint to get a subscriber by email

Migration command:

• not written, remove unsubscribed subscribers manually

[supun-io]

@Nadil-K need a careful review of the POST /subscribers endpoint, and the used services and the listeners (ConfirmationMailListener, ListRemovalListener).

[Nadil-K]

• What happened to opt_in_at? How do we handle this?

[supun-io]

• add GET /subscribers/email/:email should return lists: string[]