Stop sending milestone and labels for Projects::MergeRequestsController#show.json requests

WHY

The Projects::MergeRequestsController#show.json is one of the most requested endpoints. Ideally we should serialize just what is being used in the end of this request. Upon further investigation I've noticed that we serialize Labels and Milestones through the IssuableEntity, data which is needed only for the IssueEntity according to https://gitlab.com/gitlab-org/gitlab-ce/commit/ebf915511359c336b99c1127c5b902f8757ba5e0.

WHAT

Creates an entity to serialize solely for the merge request widget.

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• 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
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?