Monster's damage vulnerabilities, resistances and immunities should be NamedAPIResource structures

Using full references makes the content better digestible by data-consuming applications, and also brings the structure of monsters' damage immunities/etc. in line with the structure of their condition immunities.

Instead of this:
```
damage_immunities: [
"acid"
]
```

We should see this:
```
damage_immunities: [
{
"index": "acid",
"name": "Acid",
"url": "/api/damage-types/acid"
}
]
```