[schema] Let RQLConstraint inherit from BaseRQLConstraint instead of RQLVocabularyConstraint There is apparently no reason for RQLConstraint to inherit from RQLVocabularyConstraint as it gets its `repo_check` method from `RepoEnforcedRQLConstraintMixIn` and RQLVocabularyConstraint (which inherits from BaseRQLConstraint) actually only overrides that method. So simplify inheritance by removing a useless level of indirection. Adjust cw_unrelated_rql accordingly, where it was that RQLConstraint inherited from RQLVocabularyConstraint.

Latest changes 
--------------

* table of `all latest changes`_

Links below is providing useful RQL query example.

.. _all latest changes: ../view?rql=Any+M%2CX+WHERE+X+modification_date+M+ORDERBY+M+DESC+LIMIT+30