Add reply_to fields to template schemas

We're hiding the `service_letter_contact_id` column since it should
only be readable and writable using the `.reply_to` wrapper.

Schemas are defined using `fields.Method` since the fields are
represented by a property on the Template model that requires
template type to be set. When creating a template, if `reply_to`
is defined using `fields.String` it gets assigned at the same time
as `template_type` (or the order of assignments is not defined),
so the schema loader attempts to set `.reply_to` on a Template
object with a `None` `template_type`, which raises an exception.

Using `fields.Method` seems to delay `.reply_to` assignment until
the Template object is created, which means it already has a
valid type.

app/schemas.py

@@ -314,9 +314,14 @@ class NotificationModelSchema(BaseSchema):
 
 class BaseTemplateSchema(BaseSchema):
 
+    reply_to = fields.Method("get_reply_to", allow_none=True)
+
+    def get_reply_to(self, template):
+        return template.reply_to
+
     class Meta:
         model = models.Template
-        exclude = ("service_id", "jobs")
+        exclude = ("service_id", "jobs", "service_letter_contact_id")
         strict = True
 
 
@@ -339,9 +344,14 @@ class TemplateSchema(BaseTemplateSchema):
 
 class TemplateHistorySchema(BaseSchema):
 
+    reply_to = fields.Method("get_reply_to", allow_none=True)
+
     created_by = fields.Nested(UserSchema, only=['id', 'name', 'email_address'], dump_only=True)
     created_at = field_for(models.Template, 'created_at', format='%Y-%m-%d %H:%M:%S.%f')
 
+    def get_reply_to(self, template):
+        return template.reply_to
+
     class Meta:
         model = models.TemplateHistory