[CE] Resolve "Extract EE specific files/lines for some controller specs"

CE MR for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5803.

What does this MR do?

This reduces the diff between CE and EE for several files under spec/controllers.

• 35fed91d 2018-05-22 | Reduce CE/EE diff in spec/controllers/projects/project_members_controller_spec.rb [Rémy Coutable]
• 23e1330a 2018-05-22 | Reduce CE/EE diff in spec/controllers/projects/issues_controller_spec.rb [Rémy Coutable]
• 376a740f 2018-05-22 | Reduce CE/EE diff in spec/controllers/projects/imports_controller_spec.rb [Rémy Coutable]
• 4c21724a 2018-05-22 | Reduce CE/EE diff in spec/controllers/projects/group_links_controller_spec.rb [Rémy Coutable]
• e0744bc3 2018-05-22 | Reduce CE/EE diff in spec/controllers/projects/environments_controller_spec.rb [Rémy Coutable]
• c1dc6601 2018-05-22 | Reduce CE/EE diff in app/controllers/projects/clusters_controller.rb [Rémy Coutable]
• da627c72 2018-05-22 | Reduce CE/EE diff in spec/controllers/projects/branches_controller_spec.rb [Rémy Coutable]
• 4cf9adec 2018-05-22 | Reduce CE/EE diff in spec/controllers/projects/boards_controller_spec.rb [Rémy Coutable]
• ecc1131d 2018-05-22 | Reduce CE/EE diff in spec/controllers/boards/issues_controller_spec.rb [Rémy Coutable]

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

Does this MR meet the acceptance criteria?

• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #6045 (closed)