Issue #1975490 by ParisLiakos: Convert locale_custom_strings_* to settings.

core/core.services.yml

core/includes/install.core.inc

...rupal/Core/StringTranslation/Translator/CustomStrings.php

.../modules/simpletest/lib/Drupal/simpletest/WebTestBase.php

.../system/lib/Drupal/system/Tests/Common/FormatDateTest.php

...es/system/lib/Drupal/system/Tests/Menu/MenuRouterTest.php

...Drupal/system/Tests/Plugin/CacheDecoratorLanguageTest.php

...s/system/lib/Drupal/system/Tests/System/PageTitleTest.php

sites/default/default.settings.php