Email subscription management

[RaenonX]

The data structure should be something like this:

{
  key: {
    category: "post",
    type: "quest",
    id: 33,
  },
  recipients: [
    "[email protected]",
    "[email protected]"
  ]
}

key should be a type so that it can be either translate to text for the front-end to display in UI.

Also, there should be a few constant keys for the special cases below:

• All quest post
• All analysis post
• All misc post
• All tier notes
• Site announcement

Event binding

• Post publish (global only)
• Post edit (global, individual)
• Tier publish (global only)
• Tier edit (global, individual)