Move duplicate issue management to a service

app/helpers/system_note_helper.rb

app/services/issuable_base_service.rb

app/services/issues/base_service.rb

app/services/issues/duplicate_service.rb

app/services/issues/update_service.rb

app/services/quick_actions/interpret_service.rb

app/services/system_note_service.rb

app/views/shared/icons/_icon_clone.svg

...gelogs/unreleased/26372-duplicate-issue-slash-command.yml

spec/services/issues/duplicate_service_spec.rb

spec/services/issues/update_service_spec.rb

spec/services/quick_actions/interpret_service_spec.rb

spec/services/system_note_service_spec.rb