Allow filtering of entities by attributes of type "reference"

[crazyscientist]

Problem

The entity list of a schema can be filtered by all attributes in combination with different operators. But in the current implementation, this does not include attributes of type "reference".

In the UI, a user is allowed to select such an attribute/field for filtering, but is offered no operator.

In the API, no filters for these fields are know.

Expectation

Attributes of type "reference" can also be filtered; either by id or slug in combination with the usual operators.

[crazyscientist]

This is best implemented after #35...