Add parent objects to mongodb documents

[signebedi]

If we want to create child documents of parent documents...

This should be as simple as adding a parent key:value

[signebedi]

[pages] consider making children elements draggable in the UI
In #9, I think we needed to make ONLY parent objects draggable, lest we run into complexity issues. However, over time it will be worth considering how we can make the children elements draggable, too...

[signebedi]

[bug] position field broken with children elements
Currently, when we create children, it still uses the standard position of the document based on the length of the total database.

    def create(self, data, parent_id=None):
        data['created_at'] = datetime.datetime.now()
        data['last_edited'] = datetime.datetime.now()
        data['position'] = self.collection.count_documents({}) + 1  # Assign the next position
        if parent_id is not None:
            data['parent_id'] = parent_id  # Add the parent_id to the document
        return self.collection.insert_one(data).inserted_id

We need to change this such that, if parent is not null, we use different logic to determine position.