Refactor JSON templates to be generated with ActiveModelSerializers instead of Rabl

[Gargron]

Why:

• Code looks cleaner, feels like it will be cleaner than Rabl when implementing ActivityPub serializers
• I believe AMS might be more performant than Rabl

I have not yet managed to completely remove Rabl as a dependency. "Initial state" and OEmbed responses, as well as the old ActivityPub stuff that will be replaced still uses Rabl.

[Gargron]

Benchmark:

s = Status.last
Benchmark.ms { 1000.times { InlineRenderer.render(s, nil, 'api/v1/statuses/show') } }

vs.

s = Status.last
Benchmark.ms { 1000.times { InlineRenderer.render(s, nil, :status) } }

Old (Rabl): 2138.066630999674ms
New (AMS): 1626.861528999143ms

[ykzts]

This PR may be broken.

$ curl -sS -H"Authorization: Bearer ${MASTODON_ACCESS_TOKEN}" http://localhost:3000/api/v1/statuses/29 | jq '.'
{
  "id": 29,
  "created_at": "2017-07-07T06:24:50.062Z",
  "in_reply_to_id": null,
  "in_reply_to_account_id": null,
  "sensitive": false,
  "spoiler_text": "",
  "visibility": "public",
  "language": null,
  "uri": "tag:localhost:3000,2017-07-07:objectId=29:objectType=Status",
  "content": "<p>RT <span class=\"h-card\"><a href=\"http://localhost:3000/@ykzts\" class=\"u-url mention\">@<span>ykzts</span></a></span> a</p>",
  "url": "http://localhost:3000/@ykzts/29",
  "reblogs_count": 0,
  "favourites_count": 0,
  "favourited": false,
  "reblogged": true,
  "muted": false,
  "reblog": {
    "id": 28,
    "created_at": "2017-07-07T06:24:44.848Z",
    "in_reply_to_id": null,
    "in_reply_to_account_id": null,
    "sensitive": false,
    "spoiler_text": "",
    "visibility": "public",
    "language": "lb",
    "uri": "tag:localhost:3000,2017-07-07:objectId=28:objectType=Status",
    "content": "<p>a</p>",
    "url": "http://localhost:3000/@ykzts/28",
    "reblogs_count": 1,
    "favourites_count": 0,
    "favourited": false,
    "reblogged": true,
    "muted": false
  },
  "application": null,
  "account": {
    "id": 2,
    "username": "ykzts",
    "acct": "ykzts",
    "display_name": "",
    "locked": false,
    "created_at": "2017-07-07T05:17:04.201Z",
    "note": "<p></p>",
    "url": "http://localhost:3000/@ykzts",
    "avatar": "http://localhost:3000/avatars/original/missing.png",
    "avatar_static": "http://localhost:3000/avatars/original/missing.png",
    "header": "http://localhost:3000/headers/original/missing.png",
    "header_static": "http://localhost:3000/headers/original/missing.png",
    "followers_count": 1,
    "following_count": 1,
    "statuses_count": 15
  },
  "media_attachments": [],
  "mentions": [],
  "tags": []
}

missing reblog.account, reblog.application, reblog.media_attachments, reblog.mentions and reblog.tags.

[unarist]

#4095 was merged, but /api/v1/notifications seems still broken. Reblog notification includes reblog itself instead of original status, and notifications are not property ordered.

[unarist]

/api/v1/statuses/:id/unfavourite executes unfavouriting asynchronously, so it should return favourited: false regardless of current state, but it doesn't now.

Maybe this PR ignores @favourites_map?

https://github.com/tootsuite/mastodon/blob/8b2cad5/app/controllers/api/v1/statuses/favourites_controller.rb#L18