Feature: Expose report categories via an API

[ThisIsMissEm]

For now, I've just added to the /api/v2/instance endpoint, however, we may want to actually move the localisation to the server-side, which would give us an API more like: /api/v1/report_categories or /api/v2/instance/report_categories which could then negotiate locale based on requesting user.

In this WIP iteration, this looks like the following in the response:

{
  // ...ommited the rest of the instance response
  "rules": [
    {
      "id": "1",
      "text": "Test Rule",
      "hint": ""
    }
  ],
  "report_categories": [
    {
      "name": "other"
    },
    {
      "name": "spam"
    },
    {
      "name": "legal"
    },
    {
      "name": "violation"
    }
  ]
}

The violation category is dynamically present based on whether or not the server has any rules defined.