Refactor item priority composition

Allow item priorities to be grouped, such that multiple items can be considered as a single entity.

e.g.:

• Super Metroid "ball explosives", "ammo"; one of either "Bomb" or "Power Bomb" is considered "important", any more beyond that is "medium"; "Super Missiles" or "Missiles" up to a certain point are "medium", then "low".
• Link to the Past bombs (of any count, capacity upgrades, etc): first pickup may be "medium", more are "low"

Like #13 (closed), needs some sort of state tracking per-world for items received; more important that info is persisted between backend restarts.

Refactor data structure to allow thresholds and custom badges per-item (#12 (closed)), or bubble up to group reference:

{
  ...
  "items": {
    "item": {
      "ref": "<group name>",
      "received": 0 // track number received
    },
    "other item": {
      "ref: "<group name>",
      "priority": "...", // per-item override, highest precedence - ignore ref, threshold
      "badge": "...", // per item override
      "threshold": {}, // per item override
    }
  },
  "groups": {
    "<group name>": {
      "badge": ":rocket:",
      "threshold": {
        "1": "important",
        "5": "medium",
        "_": "low"
    }
  }
}