Refacto: add `get_custom_columns` to `CustomColumnsMixin`
Refacto: add `get_custom_columns` to `CustomColumnsMixin`

Small refactoring to be able to peek into custom columns without exposing the MAP_SETTINGS.

GeotrekCE/Geotrek-admin,GeotrekCE/Geotrek-admin,GeotrekCE/Geotrek-admin,GeotrekCE/Geotrek-admin