Rework Devops Adoption API

...210521073920_drop_devops_adoption_namespace_uniqueness.rb

db/schema_migrations/20210521073920

db/structure.sql

doc/api/graphql/reference/index.md

...report/devops_adoption/components/devops_adoption_app.vue

...ops_adoption/components/devops_adoption_segment_modal.vue

..._find_or_create_devops_adoption_segments.mutation.graphql

...on/graphql/queries/devops_adoption_segments.query.graphql

ee/app/finders/analytics/devops_adoption/segments_finder.rb

...analytics/devops_adoption/segments/bulk_find_or_create.rb

...ql/mutations/analytics/devops_adoption/segments/create.rb

...vers/admin/analytics/devops_adoption/segments_resolver.rb

...hql/types/admin/analytics/devops_adoption/segment_type.rb

ee/app/models/analytics/devops_adoption/segment.rb

...s/devops_adoption/segments/bulk_find_or_create_service.rb

...ices/analytics/devops_adoption/segments/common_methods.rb

...ices/analytics/devops_adoption/segments/create_service.rb

...ices/analytics/devops_adoption/segments/delete_service.rb

...lytics/devops_adoption/segments/find_or_create_service.rb

ee/spec/factories/analytics/devops_adoption/segments.rb