Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2597359 by Wim Leers: Require responses with attachments to contain the final attachment values

Issue #2559969 by alexpott: Make SafeStringInterface extend \JsonSerializable so we don't have to do so much string casting

Issue #2506581 by alexpott, Wim Leers, Fabianx, xjm, joelpittet, Cottser, dawehner: Remove SafeMarkup::set() from Renderer::doRender

Issue #2511472 by Wim Leers, Fabianx, dawehner: Refactor all usages of drupal_render()/Renderer::render() that break #2450993

Issue #2407195 by Wim Leers, Fabianx, joelpittet, lauriii, Crell: Move attachment processing to services and per-type response subclasses