Issue #2003892 by tim.plunkett, vijaycs85: Convert date formats to config entities.

core/core.services.yml

core/lib/Drupal/Core/Datetime/Date.php

core/modules/datetime/datetime.module

...etime/Plugin/field/formatter/DatetimeDefaultFormatter.php

...al/datetime/Plugin/field/widget/DatetimeDefaultWidget.php

.../datetime/lib/Drupal/datetime/Tests/DatetimeFieldTest.php

...al/field/Plugin/Type/Formatter/FormatterPluginManager.php

core/modules/locale/locale.install

core/modules/system/config/system.date.yml

core/modules/system/config/system.date_format.fallback.yml

core/modules/system/config/system.date_format.html_date.yml

...odules/system/config/system.date_format.html_datetime.yml

core/modules/system/config/system.date_format.html_month.yml

core/modules/system/config/system.date_format.html_time.yml

core/modules/system/config/system.date_format.html_week.yml

core/modules/system/config/system.date_format.html_year.yml

...s/system/config/system.date_format.html_yearless_date.yml

core/modules/system/config/system.date_format.long.yml

core/modules/system/config/system.date_format.medium.yml

core/modules/system/config/system.date_format.short.yml