Merge: Nitdoc: HTML templates related cleaning

This PR reorganizes Nitdoc modules importation for HTML templates.

The main objective is to dissociate `doc_model` from html related things.
This is the last step before template migration from old model templates to `DocComposite`.

Pull-Request: #1172
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>