Issue #2013094 by pwieck, Cottser: Remove unnecessary template_preprocess() reference from all Twig files

Issue #1363112 by mparker17, mgifford, tlattimore, Jacine, dcmouyard, bowersox, ry5n, ahdiaz: Simplify names of 'element-x' helper classes.

Issue #1898050 by duellj, Cottser, meeli, dsdart, Fabianx | c4rl: book.module - Convert PHPTemplate templates to Twig.