Issue #2335879 by alexpott, dixon_, Wim Leers, chx, mauzeh, ygerasimov, jeqq: Change SqlContentEntityStorageSchema::requiresEntityDataMigration() to ask the old storage handler if it has data rather than assuming yes unless NULL storage

Revert "Issue #2232477 by tstoeckler, a 'fliptable' mateescu, plach: Fixed Fatal when adding new fields with NOT NULL constraints in a base table that contains existing entities."

This reverts commit 08d3ebba.

Issue #2232477 by tstoeckler, a 'fliptable' mateescu, plach: Fixed Fatal when adding new fields with NOT NULL constraints in a base table that contains existing entities.

Issue #2100343 by Berdir, fago, martin107, visabhishek, sriharsha.uppuluri, Yorirou: Remove 'fieldable' key in entity definitions in favour of 'field_ui_base_route'.

Issue #2337927 by effulgentsia, plach, fago: Fixed SqlContentEntityStorage::onFieldStorageDefinition(Create|Update|Delete)() is broken for base fields.

Issue #2333113 by effulgentsia, plach: Add an EntityDefinitionUpdateManager so that entity handlers can respond (e.g., by updating db schema) to code updates in a controlled way (e.g., from update.php).

Issue #2330121 by plach, effulgentsia, fago: Replace ContentEntityDatabaseStorage::getSchema() with a new EntityTypeListenerInterface implemented by SqlContentEntityStorageSchema.

Issue #2326949 by plach, effulgentsia: Move entity-type-specific schema information from the storage class to a schema handler.

Issue #2183231 by tstoeckler, plach, jessebeach, fago, Berdir, xjm, mauzeh: Make ContentEntityDatabaseStorage generate static database schemas for content entities.