[JRO] Open user class and shove assoc's in there

There are more than a few associations tied to the user but until now
they've all been one-sided. We have the user_id FK in our models but no
reciprocating has_one or has_many tie from the parent app user.

In here we'll make those ties with a module that's included after
initialization of the engine.

```
config.after_initialize do
  Thredded.user_class.send(:include, Thredded::UserDecorator)
end
```