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.

Rollback of Issue #2138239 by damiankloip, tim.plunkett, amateescu: Use GlobIterator instead of glob. — breaks testbot.

Bah. Wrong commit message.

This reverts commit 843d7d66.

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 #2129039 by plach, jthorson, pfrenssen, nyirocsaba, Gábor Hojtsy: Integrity constraint violation when translating body field.

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

Issue #2073531 by tim.plunkett, dawehner, Berdir, catch, plach: Use current user service instead of _account, remove _account from the request object.

Issue #2133469 by tim.plunkett, damiankloip, dawehner: Replace path-based entity links with route names.

Issue #2101427 by David_Rothstein, evilehk, nod_: Follow-up: Browser validation error with Chrome on hidden required fields, even on forms that don't allow client-side validation.

Issue #2121461 by Xano: \Drupal\Core\EventSubscriber\ViewSubscriber::onHtml() does not allow controllers to return string content.

Issue #1798296 by damiankloip, Xano, David_Rothstein, Crell, dawehner: Integrate CSRF link token directly into routing system.

Issue #1810350 by plach, kfritsche, boran, YesCT: Remove menu_* entity keys and replace them with entity links.

Issue #2090541 by plopesc, yched: Avoid storing numeric target_id()'s for default values in field instance CMI files.

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.