Extending entities dont have the matching metadata

[pscheit]

When extending author from user, no STI or CTI strategy is chosen automatically. The doctrine metadata is therefore not correct

[pscheit]

• make the CTI entity abstract because otherwise we'll get: [Doctrine\ORM\Mapping\MappingException] Entity 'BaseEntity' has to be part of the discriminator map of 'BaseEntity' to be properly mapped in the inheritance hierarchy. Alter natively you can make 'BaseEntity' an abstract class to avoid this exception from occurring.
• add the discriminator map to the BaseEntity and fill it with all extending classes: note: this is tricky because we'll have to hack into "userland" here. Maybe we should find another way to provide this user data