Issue #2403817 by larowlan: Feed entity validation misses form validation logic

...lugin/Validation/Constraint/UniqueFieldValueValidator.php

core/modules/aggregator/src/Entity/Feed.php

core/modules/aggregator/src/FeedForm.php

.../src/Plugin/Validation/Constraint/FeedTitleConstraint.php

...or/src/Plugin/Validation/Constraint/FeedUrlConstraint.php

core/modules/aggregator/src/Tests/AddFeedTest.php

core/modules/aggregator/src/Tests/FeedValidationTest.php

.../user/src/Plugin/Validation/Constraint/UserMailUnique.php

.../user/src/Plugin/Validation/Constraint/UserNameUnique.php