Dereference remote replies

[tsmethurst]

This PR adds two things:

1. Allow GoToSocial to dereference remote threads by searching up and down ancestors and descendants. This is performed when a new announce comes in, for instance.
2. Allow other servers to do the same for GoToSocial, by exposing a 'replies' collection on statuses.

This PR also generally cleans up some of the federating functionality, and eliminates some code repetition where it was becoming unwieldy/silly.

A replies entry on a status now looks like this:

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "first": {
    "id": "http://localhost:8080/users/the_mighty_zork/statuses/01F8MHAMCHF6Y650WCRSCP4WMY/replies?page=true",
    "next": "http://localhost:8080/users/the_mighty_zork/statuses/01F8MHAMCHF6Y650WCRSCP4WMY/replies?only_other_accounts=false&page=true",
    "partOf": "http://localhost:8080/users/the_mighty_zork/statuses/01F8MHAMCHF6Y650WCRSCP4WMY/replies",
    "type": "CollectionPage"
  },
  "id": "http://localhost:8080/users/the_mighty_zork/statuses/01F8MHAMCHF6Y650WCRSCP4WMY/replies",
  "type": "Collection"
}

Dereferenced, the `next` entry looks like this:

```json
{
  "@context": "https://www.w3.org/ns/activitystreams",
  "id": "http://localhost:8080/users/the_mighty_zork/statuses/01F8MHAMCHF6Y650WCRSCP4WMY/replies?page=true&only_other_accounts=false",
  "items": "http://localhost:8080/users/1happyturtle/statuses/01FCQSQ667XHJ9AV9T27SJJSX5",
  "next": "http://localhost:8080/users/the_mighty_zork/statuses/01F8MHAMCHF6Y650WCRSCP4WMY/replies?only_other_accounts=false&page=true&min_id=01FCQSQ667XHJ9AV9T27SJJSX5",
  "partOf": "http://localhost:8080/users/the_mighty_zork/statuses/01F8MHAMCHF6Y650WCRSCP4WMY/replies",
  "type": "CollectionPage"
}

Note that items here is a string; when multiple items are added, it becomes an array. See go-fed/activity#139