Issue #2565895 by dawehner, stefan.r, googletorp, effulgentsia, lauriii,...

Issue #2565895 by dawehner, stefan.r, googletorp, effulgentsia, lauriii, jcnventura, David_Rothstein, catch, xjm, joelpittet, pwolanin, plach, larowlan, cilefen: Add a new :placeholder to SafeMarkup::format() for URLs that handles bad protocols