Add endpoint for tracking report per person in a given group

What does this MR do?

#10741 (closed)

Adds a new GraphQL endpoint for the GroupType to expose time logs for group users.

{
  group(fullPath: "gitlab-org") {
    name
    groupTimelogsEnabled
    timelogs(startDate:"2019-10-10", endDate: "2019-11-10"){
      edges{
        node{
          date
          timeSpent
          user {
            username
          }
          issue {
            title
            epic {
              title
            }
            milestone {
              title
            }
          }
        }
      }
    }
  }
}

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers