Issue #1900458 by Cottser, steveoliver: Use .html.twig instead of .twig file extension.

core/includes/theme.inc

core/modules/node/templates/node.html.twig

...es/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php

core/modules/system/templates/datetime.html.twig

...les/system/tests/themes/test_theme_twig/node--1.html.twig

.../modules/system/tests/themes/test_theme_twig/node--1.twig

...themes/test_theme_twig/theme_test.template_test.html.twig

core/modules/toolbar/templates/toolbar.html.twig

core/themes/engines/twig/twig.engine