Issue #2559605 by Cottser, stefan.r: Convert SafeMarkup::checkPlain() in render arrays to #plain_text

Issue #2469911 by joshi.rohit100: The edit image styles form's cancel link should be styled a a button

Issue #2457887 by prateekMehta, stefan.r, rpayanm, alexpott: Use Utility\SafeMarkup class instead of Utility\String for placeholder(), checkPlain(),format() functions

Issue #1728804 by Berdir: Introduce (Content)EntityDeleteForm and children to handle entity deletions

Issue #2378095 by Wim Leers, Devin Carlson: Convert all remaining attached individual CSS/JS assets to attached asset libraries

Issue #2339151 by EclipseGc, tim.plunkett, Gábor Hojtsy, effulgentsia: Conditions / context system does not allow for multiple configurable contexts, eg. language types

Issue #2347465 by tim.plunkett, dawehner, cilefen: Convert all instances of #type link/links to convert to use routes.

Issue #2337709 by tim.plunkett: Convert form_state_values_clean() to FormStateInterface::cleanValues().

Issue #2335659 by tim.plunkett, dawehner, effulgentsia, sun: Remove FormState ArrayAccess usage from core.

Issue #2022875 by Berdir, alexpott, Xano, swentel, tim.plunkett, vladan.me: Resolve difference between submitForm(), submit(), and save() in EntityFormController.

Issue #1938910 by JeroenT, Temoor, gnuget, pratik60, dsnopek, Brandonian | duellj: Convert image theme tables to table #type.

Issue #2308821 by tim.plunkett: Replace FormErrorInterface with $form_state->setErrorByName() and $form_state->setError().

Issue #2251223 by tim.plunkett: ConfigurableImageEffectInterface should use ConfigurablePluginInterface and PluginFormInterface.