Resolve CE to EE differences in the lib/api directory

What does this MR do?

This MR resolves all differences between CE and EE in the lib/api directory.

We'll exclude moving API parameters for now as we still need to decide on how to do this properly. This means that this MR will not entirely fix https://gitlab.com/gitlab-org/gitlab-ee/issues/9505, instead it will fix most of it.

CE backport MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25430

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/9505

TODO

• lib/api/api.rb
• lib/api/discussions.rb
• lib/api/entities.rb
• lib/api/group_boards.rb
• lib/api/groups.rb
• lib/api/helpers.rb
• lib/api/internal.rb
• lib/api/notes.rb
• lib/api/resource_label_events.rb
• lib/api/search.rb
• lib/api/services.rb
• lib/api/settings.rb
• lib/api/triggers.rb
• lib/api/variables.rb

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• Security reports checked/validated by reviewer