Issue #2042807 by tim.plunkett, pwolanin, ianthomas_uk, jhodgdon: Convert search plugins to use a ConfigEntity and a PluginBag.

This reverts commit cb41c8cf.

Issue #2163231 by jessebeach: RTL styling was jumbled while fixing the disappearing Toolbar tray box-shadow issue.

Issue #2162685 by clemens.tolboom: CKEditor's Internal::generateFormatTagsSetting() should call check_markup() with caching enabled.

Issue #1963474 by infojunkie, widukind, Cottser, joelpittet, hussainweb, nod_: Ckeditor.module - Convert theme_ functions to Twig.

Issue #1898464 by jenlampton, Cottser, joelpittet, steveoliver, Dustin Currie, shanethehat, cafuego, mr.baileys, adnen: toolbar.module - Convert theme_ functions to Twig.

Issue #1987854 by splatio, disasm, kim.pepper, vijaycs85, sidharthap, dawehner: Convert system_themes_page() to a new style controller.

Issue #2168101 by xjm: UpdateModuleHandler does not follow coding standards for switch statements and is hard to read.

Issue #2167469 by pwolanin: Test EntityViewControllerTest has the wrong descriptive text in getInfo().

Issue #2042739 by jhedstrom, bdone, dawehner, tim.plunkett: Convert system module's ControllerResolverTest to phpunit.

Issue #2159755 by damiankloip: Unit test the Drupal\serialization\EntityResolver\ChainEntityResolver class.

Issue #2041793 by mortendk, rteijeiro, LewisNyman, magnify, axe312, zorp, mdrummond: Install-page.html.twig markup and CSS.

Issue #2168113 by penyaskito, jessebeach, catch: Add leading underscore and other discouragement to drupal_add_css() and drupal_add_js().

Issue #2169067 by penyaskito: Typo in core/modules/migrate_drupal/tests/Drupal/migrate_drupal/Tests/source/d6/RoleSourceTest.php comment.

Issue #2149841 by drastik, xjm, amateescu: The 'aggregator_feed' entity type doesn't have a UUID field.

Issue #2080543 by larowlan, angel.h, grisendo, mrsinguyen: Remove unused local variables in the forum module.

Issue #2168335 by vijaycs85: Rename field.field.forum_container to field.field.forum.forum_container to match with field standard configuration.

Issue #2149827 by Xano, Mile23: Windows-1250 character encoding not supported by PHP's mbstring extension.