(cherry picked from commit 2ddd4bf6)

Issue #2429617 by Wim Leers, Fabianx, Berdir, yched, dawehner, effulgentsia, catch, borisson_, jhodgdon, martin107, torgosPizza: Make D8 2x as fast: Dynamic Page Cache: context-dependent page caching (for *all* users!)

Revert "Issue #2527126 by dawehner, Wim Leers, hass: Only send cache context/tags if frontend proxy exists"

This reverts commit 7115108c.

Issue #2408013 by Aki Tendo, alexpott, Fabianx, dawehner, Crell, jhodgdon, Wim Leers, aspilicious: Adding Assertions to Drupal - Test Tools

Issue #2509898 by dawehner, znerol, larowlan, lauriii, Aki Tendo, Wim Leers: Additional uncaught exception thrown while handling exception after service changes

Issue #2221699 by mpdonadio, mikey_p, alexpott, kim.pepper, arlinsandbulte: HTTP_HOST header cannot be trusted

Revert "Issue #2221699 by mpdonadio, mikey_p, kim.pepper, arlinsandbulte: HTTP_HOST header cannot be trusted"

This reverts commit 2c9eddf3.

Issue #2221699 by mpdonadio, mikey_p, kim.pepper, arlinsandbulte: HTTP_HOST header cannot be trusted

Issue #2327095 by tstoeckler: Document how to allow test module and theme installation in example.settings.local.php.

Issue #2251113 by alexpott, damiankloip, Crell, Wim Leers, dawehner, sun: Use container parameters instead of settings.

Issue #2321407 by astrocling | jhodgdon: Fixed Suggested setting in example.settings.local.php for render doesn't work in install.

Issue #2263059 by Bryn Vertesi, Xano: Use example.com not yoursite.com in example.settings.local.php docs

Issue #2309575 follow-up by Wim Leers: [Revert] Remove the null and memory backend definitions from core.services.yml in favour of devel module providing them instead.

Issue #2309575 by undertext | damiankloip: Remove the null and memory backend definitions from core.services.yml in favour of devel module providing them instead.