WIP: Related issues FE

What does this MR do?

• BE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1719
• FE architecture: https://gitlab.slack.com/archives/C4GFEUG90/p1492701050309816

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

Why was this MR needed?

Screenshots (if relevant)

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
• 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

Todo

• Wait for return of all related issues on every interaction, POST, DELETE
• Figure out the transient 422 Unprocessable Entity -> InvalidAuthenticityToken when interacting with some related issues endpoints
  • X-CSRF-Token is in place and vue_resource_interceptor definately fires
  • It seems like the CSRF token is getting invalidated somehow
• Wait for endpoint for fetching issue info by reference or URL
• Show errors from requests in UI
• Add tests
  • related_issues_root
  • related_issues_service
  • related_issues_store
  • Some overview rspec tests

Notes

RelatedIssue.create!(
  issue: Issue.where(
    iid: 40,
    project_id: Project.where(namespace_id: Group.where(path: "gitlab-org")[0].id, path: "gitlab-ce")[0].id
  )[0],
  related_issue: Issue.where(
    iid: 41,
    project_id: Project.where(namespace_id: Group.where(path: "gitlab-org")[0].id, path: "gitlab-ce")[0].id
  )[0]
)

What are the relevant issue numbers?

Closes #2001 (closed)