Expose target_iid in Events API

What does this MR do?

Expose target_iid in Events API

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

Since API v4, issue and MR API are requires iid. But this API returns only id yet.

Screenshots (if relevant)

Example

$ curl -s --header "PRIVATE-TOKEN XXXXXXXXXXXXXXXX" http://localhost:3000/api/v4/projects/h5bp%2Fhtml5-boilerplate/events | jq .[0]
{
  "title": null,
  "project_id": 8,
  "action_name": "opened",
  "target_id": 81,
  "target_iid": 11,
  "target_type": "Issue",
  "author_id": 1,
  "data": null,
  "target_title": "iid test",
  "created_at": "2017-08-02T06:36:06.395Z",
  "author": {
    "name": "Administrator",
    "username": "root",
    "id": 1,
    "state": "active",
    "avatar_url": "http://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80&d=identicon",
    "web_url": "http://localhost:3000/root"
  },
  "author_username": "root"
}

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

No