Use config-set models for user/group relationships.

In addition/response to #263, this uses `SentryServiceProvider` to statically inject the User and Group models as defined in the Sentry config file and use them in the `users()` and `groups()` relationships respectively.

Also, while at it, I made the pivot table name configurable as well (I needed that personally, so since we're adding customizability, why stop at the model names?). Tested in my app and working well as best as I can tell.

I'm not entirely sure if I'm doing this in the most ideal / best practice way. I just tried to emulate what was already there for `setLoginAttributeName()`. I'd be happy to fix/update based on feedback. If unit tests are also desired with this, I can throw those in as well. :)

Cheers! :beer: