Use model constructors when creating history instances

`create_history` used to initialise an empty model instance and
assign attributes to it one by one. Assigned attributes are
collected by iterating over internal state of the modified record
and stored in a dictionary, so the order in which they are set on
the history instance is undefined.

This works fine for most model instances, but recent changes to the
Template models require `template_type` to be set before `reply_to`,
otherwise Template doesn't know which column to use to persist the
`reply_to` value.

This means that we need to order the attributes when they're being
assigned. We achieved this by defining a constructor for Template
objects that assigns `template_type` first.

In order to make it work with `create_history` we replace individual
`setattr` calls with a single call to the Template constructor which
passes the entire data dictionary as keyword arguments.

This works because all persisted attributes are defined as columns
and SQLAlchemy accepts all column values as constructor arguments.

app/history_meta.py

@@ -176,7 +176,6 @@ def create_history(obj, history_cls=None):
         history_mapper = obj.__history_mapper__
         history_cls = history_mapper.class_
 
-    history = history_cls()
     obj_mapper = object_mapper(obj)
 
     obj_state = attributes.instance_state(obj)
@@ -201,7 +200,7 @@ def create_history(obj, history_cls=None):
         # not yet have a value before insert
 
         elif isinstance(prop, RelationshipProperty):
-            if hasattr(history, prop.key + '_id'):
+            if hasattr(history_cls, prop.key + '_id'):
                 foreign_obj = getattr(obj, prop.key)
                 # if it's a nullable relationship, foreign_obj will be None, and we actually want to record that
                 data[prop.key + '_id'] = getattr(foreign_obj, 'id', None)
@@ -218,9 +217,4 @@ def create_history(obj, history_cls=None):
     data['version'] = obj.version
     data['created_at'] = obj.created_at
 
-    for key, value in data.items():
-        if not hasattr(history_cls, key):
-            raise AttributeError("{} has no attribute '{}'".format(history_cls.__name__, key))
-        setattr(history, key, value)
-
-    return history
+    return history_cls(**data)