Refactor converter building functionality for improved maintainability.
Added substitution_mixin.py with the new SubstitutionMixin abstract model.
Updated ir.actions.report to inherit from SubstitutionMixin
and implement _get_template to return redner_tmpl_id.
Updated mail.template to inherit from SubstitutionMixin,
replacing its simpler action_get_substitutions with the
mixin's implementation, and added _get_template.