Merge branch '353473-add-support-for-relating-epics-service' into 'master'

app/services/issuable_links/create_service.rb

app/services/issue_links/create_service.rb

app/services/system_note_service.rb

app/services/system_notes/issuables_service.rb

...controllers/groups/epics/related_epic_links_controller.rb

ee/app/serializers/epics/related_epic_entity.rb

ee/app/services/ee/issuable_links/create_service.rb

ee/app/services/ee/issue_links/create_service.rb

ee/app/services/epics/related_epic_links/create_service.rb

ee/app/services/epics/related_epic_links/destroy_service.rb

ee/config/routes/group.rb

ee/spec/controllers/projects/issues_controller_spec.rb

ee/spec/fixtures/api/schemas/entities/related_epic.json

...quests/groups/epics/related_epic_links_controller_spec.rb

ee/spec/serializers/epics/related_epic_entity_spec.rb

ee/spec/services/ee/issue_links/create_service_spec.rb

ee/spec/services/ee/system_notes/issuables_service_spec.rb

.../services/epics/related_epic_links/create_service_spec.rb

...es/services/issuable_links/create_links_with_link_type.rb

locale/gitlab.pot