Fixes SlugField with no_dereference

[noirbizarre]

This one was tricky !!!

This PR fixes the udata search index command and more specificaly the .no_dereference() usage with SlugField.

Root cause

In MongoEngine 0.16, no_reference() behavior change to use copy.deepcopy on fields (sadly Mongoengine make extensive use of copy.deepcopy which is bad for performances).
On the other side, the SlugField need to keep a reference to its owning instance to be able to properly slugify on save.
As a consequence, copy.deepcopy was trying to copy the SlugField.instance field which is a model and cannot be deepcopied easily

Fix

copy.deepcopy allows to override the deepcopy behavior by implementing a __deepcopy__(self, memo) method. This is done in this PR by making a proper new SlugField instance

Future thought

The extensive use of copy.deepcopy might have a bad impact on performance. In order to improve that, we might need to implement some custom __deepcopy__(self, memo) methods on other objects because this is not done by MongoEngine itself.
As flask-restplus also make use of copy.deepcopy, implementing custom __deepcopy__(self, memo) methods on models might improve performance both on DB and API sides.