Issue #2451665 by dawehner: Don't rebuild the route on ModuleInstaller::install() (30% installer speedup)

Revert "Revert "Issue #2457653 by Gábor Hojtsy: System.site langcode is both used as a file language code and a site language code""

This reverts commit 079a52b4.

Revert "Issue #2457653 by Gábor Hojtsy: System.site langcode is both used as a file language code and a site language code"

This reverts commit 364a6e9c.

Issue #2457653 by Gábor Hojtsy: System.site langcode is both used as a file language code and a site language code

Issue #356399 by dawehner, beejeebus, catch, sidharrell, klausi, nlisgo, Josh Waihi, Berdir, martin107: Optimize the route rebuilding process to rebuild on write

Issue #2442255 by swentel, larowlan, rteijeiro, Wim Leers: Changing text formats on a field makes it impossible to edit

Issue #2329781 by davidhernandez, sidharthap: Move CKEditor toolbar classes from preprocess to templates

Issue #2415855 by emma.maria: A forgotten reference to list.css in Bartik's info file is giving core a 404 error

Issue #2368797 by Wim Leers, dawehner, rteijeiro: Optimize ajaxPageState to keep Drupal 8 sites fast on high-latency networks, prevent CSS/JS aggregation from taking down sites and use HTTP GET for AJAX requests

Issue #2064379 by tadityar, bneil: Remove ckeditor-iframe.css and load relevant Bartik CSS files for CKEditor's iframe mode

Issue #2350011 by rpayanm, Palashvijay4O, xq1003, zealfire, yoroy, arunkumark: Too small/large font sizes for body text in CKEditor

Issue #2392319 by alexpott, effulgentsia: Config objects (but not config entities) should by default be immutable

Issue #2318237 by Gábor Hojtsy, bserem, no_angel, Wim Leers: CKEditor translates its user interface even if interface translation is turned off

Issue #2395395 by alexpott, effulgentsia, Gábor Hojtsy: TestBase lacks a config method to be used consistently in tests

Issue #2382543 by rpayanm, ianthomas_uk: Remove usage of drupal_html_class() and drupal_clean_css_identifier()

Issue #2183983 by Gábor Hojtsy, vijaycs85, Berdir, Wim Leers, webflo, alexpott: Find hidden configuration schema issues

Issue #2391245 by Gábor Hojtsy, Wim Leers: Resolve remaining misc issues with configuration schema fails

Issue #2385111 by tadityar, Wim Leers: Bartik's CKEditor iframe stylesheet is not being loaded, hence image captions look broken

Issue #2381303 by tibbsa, rpayanm, hussainweb, mglaman: Clean-up CKEditor module test members — ensure property definition and use of camelCase naming convention

Issue #2350327 by PieterDC, Dave Reid: editor.module should use the same data- attributes as entity_embed.module uses

Issue #2324055 by dawehner, cilefen, znerol: Split up the module manager into runtime information and extension information

Issue #2328111 by dawehner, martin107, neclimdul: Replace most instances of the DRUPAL_ROOT constant with the app.root container parameter.

Issue #2361681 by Wim Leers: drupal_render(): invert second argument ($is_recursive_call -> $is_root_call) => more strict, better DX/TX.

Issue #2340667 by YesCT, Pedro Lozano, alexpott, martin107: Protect Drupal\Core\Language\Language::id, and use getId().

Issue #2246675 by rpayanm, johnstorey, holly.ross.drupal | mgbellaire: Remove all unnecessary 'template' lines in hook_theme() declarations.

Issue #2070737 by herom, EllaTheHarpy, alexpott: Change values of LanguageInterface::DIRECTION_(LTR/RTL) to ('ltr'/'rtl').

Issue #2270251 by yvesvanlaer | Bojhan: Unwanted box around CKEditor, i.e. our CSS overrides no longer work (presumably since CKEditor 4.4 upgrade).