Use service model for the copy page

The view here is rebuilding a pseudo-service object. Now that service
objects have templates it’s cleaner to use the actual service object.

Requires a small change to the `templates_by_type` method so that it can
filter by one or many template types (a user should be able to copy any
template whose type is enabled for their service, and the service
they’re copying from).

app/models/service.py

@@ -81,9 +81,11 @@ class Service():
         ]
 
     def templates_by_type(self, template_type):
+        if isinstance(template_type, str):
+            template_type = [template_type]
         return [
             template for template in self.templates
-            if template_type in {'all', template['template_type']}
+            if set(template_type) & {'all', template['template_type']}
         ]
 
     @property