Revert "Issue #1954892 by dawehner, tim.plunkett, David_Rothstein, effulgentsia: Replace 'theme callback' and hook_custom_theme() with a clean theme negotiation system."

This reverts commit fc04601c.

Issue #2004086 by dawehner, pwolanin, katbailey, tim.plunkett, vijaycs85, fabpot: The Request service must be synthetic.

Issue #1954892 by dawehner, tim.plunkett, David_Rothstein, effulgentsia: Replace 'theme callback' and hook_custom_theme() with a clean theme negotiation system.

Issue #2118703 by Wim Leers, amateescu: Introduce #post_render_cache callback to allow for personalization without breaking the render cache.

Issue #2093167 by johnnydarkko, thedavidmeister, Vibjerg: Deprecate check_plain(), call \Drupal\Component\Utility\String::checkPlain() directly instead.

Issue #1886448 by dawehner, sun, Berdir, ParisLiakos: Rewrite the theme registry into a proper service.

Issue #2084463 by dawehner, pwolanin, David_Rothstein, tim.plunkett, Xano: Convert contextual links to a plugin system similar to local tasks/actions.

Issue #2019055 by plach, fago, berdir, yched, Gábor Hojtsy, kfritsche, jibran: Switch from field-level language fallback to entity-level language fallback.

Revert "Issue #2019055 by plach, fago, kfritsche: Switch from field-level language fallback to entity-level language fallback."

This reverts commit 46942cd3.

Issue #2019055 by plach, fago, kfritsche: Switch from field-level language fallback to entity-level language fallback.

Issue #2066535 by tim.plunkett, jsst, Berdir: Add EntityManagerInterface and stop typehinting with EntityManager.

Issue #2048223 by dawehner, ParisLiakos, herom, fubhy, damiankloip, vijaycs85, joelpittet, tim.plunkett: Add $account argument to AccessCheckInterface::access() method and use the current_user() service.

Issue #2109065 by ianthomas_uk, jhodgdon: Fixed Remove variable_get() and variable_set() calls from API documentation.

Issue #2099577 by thamas, nod_, yoroy, Gábor Hojtsy: First installer page has initially needless description for how translations are handled.

Issue #2057259 by alexpott, cweagans, swentel: Fixed Malformed modulename.info.yml file causes fatal error.

Issue #2113015 by Wim Leers, amateescu: Add drupal_merge_attached() function, to merge #attached assets.

Issue #2102125 by dawehner, tim.plunkett, neclimdul, pwolanin, YesCT, damiankloip, Tim Bozeman: Big Local Task Conversion.

Issue #1982256 by mortendk, rteijeiro, tim.plunkett, LewisNyman, tlattimore, yannickoo, ry5n: Clean up html.html.twig markup.

Issue #2046367 by jhodgdon, amateescu, larowlan, juanolalla: Menu routing docs do not match what the code is doing.

This reverts commit 7b08fd63.