Issue #2554889 by Berdir, Cottser, Daniel_Rose, JeroenT, RavindraSingh, Wim...

Issue #2554889 by Berdir, Cottser, Daniel_Rose, JeroenT, RavindraSingh, Wim Leers, YesCT, akalata, alexpott, alimac, aneek, catch, cdulude, chx, cilefen, cmanalansan, crowdcg, cwells, davidhernandez, dawehner, dtraft, effulgentsia, harjotsingh, hass, ianthomas_uk, joelpittet, josephdpurcell, justAChris, kbaringer, lauriii, lduerig, leslieg, lokapujya, mpdonadio, peezy, pwolanin, realdream, sclapp, stefan.r, tim.plunkett, webchick, xjm: Remove SafeMarkup::set() from the codebase.