Default polymorphic serializer

[silentninja]

Related to #944

Adds support for default serializer to mathesar.api.serializers.shared_serializers.ReadWritePolymorphicSerializerMappingMixin.

Default serializer is needed to provide a common set of fields like show_fk_preview for all supported column types without having to declare a serializer for each column type.

The PR also adds show_fk_preview field as a display option for all the column types.

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[dmos62]

@silentninja looks good; can you resolve merge conflicts?