Issue #1825952 by Fabianx, joelpittet, bdragon, heddn, chx, xjm, pwolanin, mikey_p, ti2m, bfr, dags, cilefen, scor, mgifford: Turn on twig autoescape by default

Issue #2212309 by joelpittet: Drupal\Core\Template and twig.engine docs, coding standards, and unused code cleanup.

Issue #2221535 by Cottser, joelpittet: TwigNodeVisitor's priority is too early and breaks some filters and macros.

Issue #2114563 by joelpittet, martin107, Cottser: Remove TwigReference with the help of 'without' filter replacing 'show'/'hide' functions.

Issue #1696786 by Fabianx, jenlampton, stevector, steveoliver, jwilson3, amateescu, chx: Integrate Twig into core: Implementation issue.