Issue #1987818 by jlbellido, MattDanger, disasm, vijaycs85: Convert system_date_format_language_overview_page() to a new style controller.

Issue #1987810 by nick_schuch, dawehner, vijaycs85, tim.plunkett: Convert system_admin_config_page() to a new style controller.

Issue #2010086 by helga.chb, somepal, jenlampton, IshaDakota: Replace theme() with drupal_render() in system module for  theme_system_themes_page().

Issue #1668866 by ParisLiakos, aspilicious, tim.plunkett, pdrake, g.oechsler, dawehner, Berdir, corvus_ch, damiankloip, disasm, marcingy, neclimdul: Replace drupal_goto() with RedirectResponse.

Issue #1363112 by mparker17, mgifford, tlattimore, Jacine, dcmouyard, bowersox, ry5n, ahdiaz: Simplify names of 'element-x' helper classes.

Issue #1861702 by echoz, rteijeiro, LewisNyman, mcjim: Add a responsive grid to the Appearance page.

Issue #1987824 by nick_schuch, vijaycs85, ParisLiakos: Convert system_php() to a new style controller.

Issue #1957154 by joates, ayelet_Cr, xjm | Berdir: Replace calls to drupal_container()->get('module_handler()') service with Drupal::moduleHandler().

This reverts commit c4e52bb6.

Issue #1987850 by oenie, vijaycs85, cosmicdreams, amateescu: Convert system_theme_disable() to a new style controller.

Issue #1946454 by andypost, kim.pepper, tim.plunkett, ParisLiakos: Convert all confirm_form() in system.module and system.admin.inc to the new form interface and convert route.

Issue #1925660 by ACF, tim.plunkett, mtift, heddn, Cottser: Convert system's system_config_form() to SystemConfigFormBase.

Issue #1712250 by alexpott, n3or, cam8001, vijaycs85, Albert Volkman, jibran, heyrocker, chx: Convert theme settings to configuration system.

Issue #1712258 by alexpott, dagmar, Shuairan, disasm, slv_: Simplify image toolkit form logic, removing unneeded checks.

Issue #625958 by slashrsm, quicksketch, eojthebrave, droplet, danlinn, czigor: Support Uploading Multiple Files for HTML5 Browsers via #multiple attribute.

Issue #1664844 by alexpott, cweagans, heyrocker, eojthebrave, ACF, Berdir: Convert image toolkits into plugins.

Issue #1829224 by alexpott, pfrenssen, vijaycs85, lirantal: Convert the 'theme_default()' variable to CMI.

Issue #1853086 by katbailey, msonnabaum, alexpott: Remove cache.page.enabled in favor of an explicit internal cache setting.