TemplateHistory objects need to be connected to the template's
TemplateRedacted record. This requires setting up a new SQLAlchemy
relationship to allow accessing redact_personalisation from
TemplateHistory instances.
We can avoid creating a foreign key in this case by setting up an
explicit `primaryjoin` expression. Since TemplateHistory.id is
created from Template.id and TemplateRedacted.template_id is
already a foreign key to Template.id the foreign key should always
be valid even without a DB constraint.