Model compatibilities

* Attr models' get_model_attrs returns only defined attrs
* Dropped Database models' get_model_attrs
Basic models, to_string

* Added _toString methods
Templates: Array support to_html

* Added support for arrays to to_html
User group, custom to_html

* Added custom to_html to user groups
[fix] Template: to_html

* Fixed recursion in template::to_html
Contact to_html

* Added custom contact to_html