Provide issue and merge request sidebar information through GraphQl

Issue no longer valid, replaced with https://gitlab.com/gitlab-org/gitlab-ee/issues/20718 and https://gitlab.com/gitlab-org/gitlab-ee/issues/20829

All information needed for the issue / merge request sidebars should available through the GraphQL API.

This is also in support of realtime sidebars https://gitlab.com/gitlab-org/gitlab-ee/issues/17589

Issues

Currently the issue sidebar shows the following information:

• Assignee
• Epic
• Milestone
• Time Tracking
• Due Date
• Labels
• Weight
• Confidentiality
• Lock issue
• Participants
• Notifications/subcribed
• Reference

The issue GraphQL API returns quite a few attributes, but only the checked ones above are currently returned.