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

Issue #2576533 by alexpott, stefan.r, Wim Leers, dawehner, xjm, effulgentsia, catch: Rename SafeStringInterface to MarkupInterface and move related classes

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 #2289917 by Wim Leers, mdrummond, lauriii, Manuel Garcia, emma.maria, Scionar, davidhernandez: Convert "messages" page element into blocks

Issue #2347469 by mondrake, Wim Leers, larowlan, kattekrab, mnico, tadityar: Rendering forms in AjaxResponses does not attach assets automatically