Move model to a trait

[GromNaN]

Fix #2120

By moving all the logic to a trait, it make it possible to reuse entities provided by community projects. Example: https://github.com/z-song/laravel-admin/blob/master/src/Auth/Database/Administrator.php

The properties $primaryKey and $keyType can't be in the trait because PHP forbid declaring a property in a trait that is different (type and value) than the property declared in the parent class.

I have mixed feelings about this change, as it breaks the link with the overloaded parent methods. It would probably be better to make the change in each community-package when necessary.

[GromNaN]

Currently, the Model class is a marker for MongoDB models. If a trait is used, an other marker will be necessary.
https://github.com/mongodb-php/laravel-mongodb/blob/f06f7b3dca88cfd5cc6a823ff01051b4a46ca10a/src/Eloquent/HybridRelations.php#L145