Clean up method visibility in `SystemNoteService`
The following methods in SystemNoteService
are underneath private
but are part of its public interface:
change_task_status
noteable_moved
Similarly, the following methods should be private but aren't, because they're class methods:
notes_for_mentioner
create_note
cross_reference_note_prefix
cross_reference_note_content
new_commit_summary
existing_commit_summary
They should be marked private using private_class_method
instead.
Related: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4084