Add custom Rubocop cop that validates presence of permission checks in services
See https://gitlab.com/gitlab-org/gitlab-ce/issues/15450 and discussion starting from https://gitlab.com/gitlab-org/gitlab-ce/issues/15450#note_5020162.
Adding Rubocop cop that would check whether there are permission checks in service class located under /services
and yield an offense if there are not, may be a good first step.