Issue #2407195 by Wim Leers, Fabianx, joelpittet, lauriii, Crell: Move attachment processing to services and per-type response subclasses

Issue #2478119 by znerol, andypost: Replace references to Session Manager with references to Session

Issue #1818692 by mbrett5062, lokapujya, David_Rothstein, blacklight4: Improve the maintenance page error message (rollback)

Issue #2451085 by dawehner, Arla: _drupal_log_error() passes NULL to ThemeManager::setActiveTheme(), violating its typehint

Issue #2317913 by jhedstrom: Early error handling can result in fatal error (Call to a member function get() on a non-object)

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

Issue #1825952 by Fabianx, joelpittet, bdragon, heddn, chx, xjm, pwolanin, mikey_p, ti2m, bfr, dags, cilefen, scor, mgifford: Turn on twig autoescape by default

Issue #2016629 by larowlan, neclimdul, sun, alexpott, jibran, ParisLiakos, donquixote, effulgentsia, msonnabaum: Refactor bootstrap to better utilize the kernel.

Revert "Issue #2016629 by larowlan, neclimdul, sun, alexpott, jibran, ParisLiakos, donquixote, effulgentsia, msonnabaum: Refactor bootstrap to better utilize the kernel."

This reverts commit cda051c4.

Issue #2016629 by larowlan, neclimdul, sun, alexpott, jibran, ParisLiakos, donquixote, effulgentsia, msonnabaum: Refactor bootstrap to better utilize the kernel.

Issue #2089433 by ianthomas_uk, grom358, herom, thedavidmeister, visabhishek, longwave, Sweetchuck: Remove uses of deprecated XSS filter functions.

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

Broke update.php.

This reverts commit 10b03890.

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

Issue #2187735 by ianthomas_uk, Sutharsan, andypost: Add removal information to docblock of all @deprecated functions.

Issue #2194111 by sun: Error handler throws exception when service container is not (fully) available yet.

Issue #2171683 by sun, alexpott, tstoeckler, larowlan: Remove all Simpletest overrides and rely on native multi-site functionality instead.

Revert "Issue #2171683 by sun, tstoeckler, larowlan: Remove all Simpletest overrides and rely on native multi-site functionality instead."

This reverts commit 91db25eb. Broke web
tests.

Issue #2171683 by sun, tstoeckler, larowlan: Remove all Simpletest overrides and rely on native multi-site functionality instead.

Issue #2176105 followup by sun: Installer catches exceptions and manually re-prints them; does not use error/exception handler.

Issue #2176105 by sun: Installer catches exceptions and manually re-prints them; does not use error/exception handler.

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

This reverts commit 127b54a6.