Simplify code around (cross)-references

app/controllers/projects/issues_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/models/concerns/mentionable.rb

app/models/concerns/participable.rb

app/models/note.rb

app/services/git_push_service.rb

app/services/issues/create_service.rb

app/services/issues/update_service.rb

app/services/merge_requests/create_service.rb

app/services/merge_requests/update_service.rb

lib/gitlab/reference_extractor.rb

spec/models/concerns/mentionable_spec.rb

spec/services/git_push_service_spec.rb

spec/support/mentionable_shared_examples.rb