Issue #2571647 by mpdonadio, joelpittet, pjonckiere: Create DateFormatterInterface

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

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

...eld/Plugin/Field/FieldFormatter/TimestampAgoFormatter.php

.../Field/Plugin/Field/FieldFormatter/TimestampFormatter.php

...odules/aggregator/src/Controller/AggregatorController.php

...ator/src/Plugin/aggregator/processor/DefaultProcessor.php

core/modules/comment/src/Form/CommentAdminOverview.php

...modules/config_translation/src/FormElement/DateFormat.php

core/modules/contact/src/MessageForm.php

...src/Plugin/Field/FieldFormatter/DateTimeFormatterBase.php

.../Plugin/Field/FieldFormatter/DateTimeTimeAgoFormatter.php

core/modules/datetime/src/Plugin/views/filter/Date.php

core/modules/dblog/src/Controller/DbLogController.php

core/modules/node/src/Controller/NodeController.php

core/modules/node/src/NodeListBuilder.php

core/modules/system/src/DateFormatListBuilder.php

core/modules/system/src/Form/CronForm.php

core/modules/system/src/Form/DateFormatDeleteForm.php

core/modules/system/src/Form/DateFormatFormBase.php

core/modules/system/src/Form/FileSystemForm.php