Fix schema creation for self referencing many to many association

[X-Coder264]

Subject

I am targeting this branch, because it's a bug fix.

We've tried to upgrade from 1.7 to 1.9 but our CI was failing with the following exception:

Doctrine\DBAL\Schema\Exception\TableDoesNotExist: There is no table with name "foo" in the schema.
/app/vendor/doctrine/dbal/src/Schema/Exception/TableDoesNotExist.php:16
/app/vendor/doctrine/dbal/src/Schema/SchemaException.php:68
/app/vendor/doctrine/dbal/src/Schema/Schema.php:188
/app/vendor/sonata-project/entity-audit-bundle/src/EventListener/CreateSchemaListener.php:102

Further debugging showed that the issue was introduced in #509 as the case of a self referencing many to many association which uses class table inheritance was not taken into consideration in the create schema listener class so it currently tries to create the revision/audit table for the related table which does not yet exist at that point. The fix was to execute that logic (if we run into that case) in the post schema generation event instead where we are sure that all schema tables are created so we can then safely create the revision table for them.

Changelog

### Fixed
- Schema creation for self referencing many to many association with class table inheritance

[VincentLanglet]

Does it solve #522 or is it a different issue ?

[X-Coder264]

#522 seems to be about a NotNullConstraintViolationException exception that gets thrown while the audit records are being recorded, while this issue is about a TableDoesNotExist exception which gets thrown during schema creation (so it blocks any further testing for us of the many to many auditing feature).

[VincentLanglet]

Thanks