Issue #2273925 by larowlan, aneek, lauriii, mikey_p, joelpittet, dimaro,...

Issue #2273925 by larowlan, aneek, lauriii, mikey_p, joelpittet, dimaro, Fabianx, xjm, jaredsmith, effulgentsia, lokapujya, iMiksu, chx, YesCT, googletorp, dawehner, Wim Leers, Cottser: Ensure #markup is XSS escaped in Renderer::doRender()