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

CE backport for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5739.

What does this MR do?

• 8d024ba7 2018-05-16 | Backport changes from EE to minimize the CE/EE diff in Projects::Settings::IntegrationsController [Rémy Coutable]
• 6d81905f 2018-05-16 | Introduce a new Keys::DestroyService service [Rémy Coutable]
• c75ae237 2018-05-16 | Add :weight as an allowed serializable field in app/controllers/boards/issues_controller.rb [Rémy Coutable]

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

I don't think so.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• 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
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

CE backport for https://gitlab.com/gitlab-org/gitlab-ee/issues/6020.