Konova filter mixins

* refactors konova/mixins.py into individual files in konova/mixins/...

compensation/filters.py

@@ -10,7 +10,7 @@ from django.utils.translation import gettext_lazy as _
 from django import forms
 from django.db.models import QuerySet, Q
 
-from konova.filters.mixins import ConservationOfficeTableFilterMixin
+from konova.filters.mixins.office import ConservationOfficeTableFilterMixin
 from konova.filters.table_filters import QueryTableFilter, CheckboxTableFilter, SelectionTableFilter, AbstractTableFilter