Backport improved spec/controllers/projects/boards_controller.rb and split EE/CE specs

The following discussion from !2317 (merged) should be addressed:

• @reprazent started a discussion:

  I've separated some EE logic out of EE files.

  The app/controllers/projects/boards_controller.rb is identical to the CE version by doing this.

  I've split up the spec/controllers/projects/boards_spec.rb into the logic that is on CE en what is on EE. However, they have diverged. So they are likely (however less than before) to cause conflicts in the future.

  We'd need to move the EE specific spec logic for Projects::BoardsController#index & Projects::BoardsController#show into spec/controllers/ee/projects/boards_controller.rb however, some of the tests in EE have been improved, so those improvements would need to be ported to CE.

  The last part seemed to drag me a bit too far, I'd rather create a new technical debt Issue for that.