Revert "Issue #2218039 by sun, dawehner: Render the maintenance/install page like any other HTML page."

Broke update.php.

This reverts commit 10b03890.

Issue #2228921 by sun: Consolidate system.module + system.theme + system.theme.disabled into core.extension config.

Issue #2166915 by ianthomas_uk, sun | plach: Remove uses of deprecated language functions in tests and procedural code.

Issue #2192649 by sun, chakrapani, sidharthap: Remove drupal_set_title() from installation and update process.

Issue #2045921 by juanolalla, Xano, Sutharsan, InternetDevels, ianthomas_uk: Replace all module_invoke() deprecated function calls.

Revert "Issue #2045921 by juanolalla, Xano, Sutharsan, InternetDevels: Replace all module_invoke() deprecated function calls."

This reverts commit 98cdbfec.

Issue #2045921 by juanolalla, Xano, Sutharsan, InternetDevels: Replace all module_invoke() deprecated function calls.

Issue #2168011 by xjm, jessebeach, Damien Tournoud, znerol, Xano: Remove all 7.x to 8.x update hooks and disallow updates from the previous major version.

Issue #2167109 by Berdir, sun, alexpott, ACF, acrollet, adamdicarlo, Albert Volkman, andreiashu, andyceo, andypost, anenkov, aspilicious, barbun, beejeebus, boombatower, cam8001, chriscalip, chx, cosmicdreams, dagmar, damiankloip, dawehner, deviance, disasm, dixon_, dstol, ebrowet, Gábor Hojtsy, heyrocker, Hydra, ianthomas_uk, japicoder, jcisio, jibran, julien, justafish, jvns, KarenS, kbasarab, kim.pepper, larowlan, Lars Toomre, leschekfm, Letharion, LinL, lirantal, Lukas von Blarer, marcingy, Mike Wacker, mrf, mtift, mtunay, n3or, nadavoid, nick_schuch, Niklas Fiekas, ParisLiakos, pcambra, penyaskito, pfrenssen, plopesc, Pol, Rok Žlender, rvilar, swentel, tim.plunkett, tobiasb, tsvenson, typhonius, vasi1186, vijaycs85, wamilton, webchick, webflo, wizonesolutions, xjm, yched, YesCT, znerol: Remove Variable subsystem.

Issue #2029537 by Xano, InternetDevels, -enzo- | tsphethean: Replace drupal_sort_weight() with SortArray::sortByWeightElement() and remove drupal_sort_weight().

Issue #1862202 by plach, Berdir, katbailey, ParisLiakos, alexpott, chx, sun, larowlan, Gábor Hojtsy, cosmicdreams, vijaycs85, YesCT, penyaskito, andypost, Albert Volkman, joelpitett: Objectify the language system.

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

Issue #2045931 by PerthSam152, LinL, adsw12, joelpittet: Replace all module_exists() deprecated function calls.

This reverts commit cc84f390 - bad
commit message.

Issue #2089351 by thedavidmeister, meba: Convert all calls to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in core/includes.

Issue #2100329 by cosmicdreams: Use \Drupal instead of Drupal to make IDEs and static code analyse tools happy.

Issue #1969572 by neclimdul, larowlan, hussainweb, dawehner, damiankloip, ParisLiakos, pwieck, Berdir, socketwench: Make Uuid a service.

Issue #1199946 by alexpott, fubhy, bojanz, steinmb, chx, catch: Fixed Disabled modules are broken beyond repair so the 'disable' functionality needs to be removed.

Issue #2084339 by alexpott: Fixed The configuration value system.maintenance.enabled should be moved to the state system.

Issue #1827112 by alexpott, vijaycs85, dawehner, cspitzlay, cam8001: Convert 'install_profile()' variable to Settings.

This reverts commit 8ce27f22.

Issue #2045923 by drupalrv, tim.plunkett: Replace all module_invoke_all() deprecated function calls in procedural code.

Issue #1998204 by tim.plunkett: Fixed config_install_default_config() is not safe to use in hook_update_N().

Issue #1856976 by andypost, Soul88, ACF, aspilicious, vijaycs85, dawehner: Convert language_count() to the state system.