[FIX] mgmtsystem_evaluation: support model_field_char option in reference widget to avoid access to ir.model

[kluna1998]

fixing:

• AttributeError: 'mgmtsystem.evaluation.template' object has no attribute '_get_mail_thread_data'

• ValueError: Invalid field mail.activity.type.res_model_id in leaf ('res_model_id', '=', 421)

• reference field configured with model_field fails to render due to access denied errors when trying to read ir.model -> _fetchModelFieldReference using BasicModel.include, and add support for model_field_char as a fallback.

@etobella