Issue #2569069 by stefan.r, mr.baileys, alexpott, lauriii: Replace TranslationWrapper with TranslatableString and deprecate TranslationWrapper

Issue #2557113 by stefan.r, alexpott, dawehner, joelpittet, cilefen, lauriii, plach, catch, chx, xjm: Make t() return a TranslationWrapper object to remove reliance on a static, unpredictable safe list

Issue #2562487 by stefan.r, chx, tim.plunkett, dawehner, alexpott: Get better error reporting from __toString

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

Issue #2338081 by pwolanin, dawehner, effulgentsia, kgoel, vijaycs85, mpdonadio, chx, brandon.holtsclaw, alexpott, Gábor Hojtsy, Fabianx, catch, YesCT, JvE: Local Tasks, Actions, and Contextual links mark strings from derivatives (or alter hooks) as safe and translated

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

Issue #2212069 by Gábor Hojtsy, pjonckiere, rteijeiro, cilefen: Non-English Drupal sites get default configuration in English, edited in English, originals not actually used if translated

Issue #2244447 by alexpott, sun, Berdir: Translation of low-level info/annotations leads to circular dependencies.