Code Review Analytics MVC - API Implementation

Summary

Provide an implementation of API for Code Review analytics. See a mock for required fields and design mockup for more details.

Technical details

• PERCENTILE CALCULATION IS OUT OF SCOPE of this ticket.
• review_time is a number of hours in review
• API should require project_id to be set and should return results scoped to specified project.
• API should be able to filter results by 1+ label and\or a milestone.
• Please create separate analytics grape entities for representation. try to keep response simple.
• Please double check DB and Gitaly performance.