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

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 #2343035 by fago, dawehner, larowlan, ianthomas_uk, cilefen, hussainweb, Miroling, jibran, beejeebus, dashaforbes, webmozart, klausi: Upgrade validator integration for Symfony versions 2.5+

Issue #2412509 by kgoel, pwolanin, Wim Leers, larowlan, effulgentsia, dawehner, xjm: Change LinkItem.uri to type 'uri' rather than 'string' and introduce user-path: scheme