TranslationManager->formatPlural() returns a PluralTranslatableMarkup

In #2557113: Make t() return a TranslationWrapper object to remove reliance on a static, unpredictable safe list we made TranslationManager->formatPlural() return a TranslatableMarkup object. To be consistent we now return a PluralTranslatableMarkup from TranslationManager->formatPlural().

Previously plural strings could be created from previously translated strings (such as those from config) using TranslationInterface::formatPluralTranslated(). This has been replaced by using the PluralTranslatableMarkup::createFromTranslatedString() static method.

See the change record https://www.drupal.org/node/2564451 for special changes that might be necessary now that plural strings are an object.